Fundamentals of Soil Science 8Th Edition (1991)

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FUNDAMENTALS OF SOIL SCIENCE FUNDAMENTALS OF SOIL SCIENCE

EIGHTH EDITION

HENRY D. FOTH Michigan State University

JOHN WILEY SONS New York Chichester Brisbane Toronto Singapore Cover Photo Soil profile developed from glacio-fluvial sand in a balsam fir-black spruce forest in the Laurentian Highlands of Quebec, Canada. The soil is classified as a Spodosol (Orthod) in the United States and as a Humo-Ferric Podzol in Canada.

Copyright © 1943, 1951 by Charles Ernest Millar and Lloyd M. Turk Copyright © 1958, 1965, 1972, 1978, 1984, 1990, by John Wiley & Sons, Inc. All rights reserved. Published simultaneously in Canada. Reproduction or translation of any part of this work beyond that permitted by Sections 107 and 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons.

Library of Congress Cataloging In Publication Data: Foth, H. D. Fundamentals of soil science / Henry D. Foth.-8th ed. p. cm. Includes bibliographical references. ISBN 0-471-52279-1 1. Soil science. I. Title. S591.F68 1990 631.4-dc20 90-33890 CIP Printed in the United States of America 1098765432 Printed and bound by the Arcata Graphics Company PREFACE

The eighth edition is a major revision in which and water flow is discussed as a function of the there has been careful revision of the topics hydraulic gradient and conductivity. Darcy's Law covered as well as changes in the depth of cover- is used in Chapter 6, "Soil Water Management," age. Many new figures and tables are included. in regard to water movement in infiltration, drain- Summary statements are given at the ends of the age, and irrigation. Chapter 6 also covers dis- more difficult sections within chapters, and a posal of sewage effluent in soils and prescription summary appears at the end of each chapter. athletic turf (PAT) as an example of precision Many nonagricultural examples are included to control of the water, air, and salt relationships in emphasize the importance of soil properties when soils used for plant growth. "Soil Erosion," Chap- soils are used in engineering and urban settings. ter 7, has been slightly reorganized with greater The topics relating to environmental quality are emphasis on water and wind erosion processes. found throughout the book to add interest to many Chapters 8 and 9, "Soil Ecology" and "Soil Or- chapters. Several examples of computer applica- ganic Matter," are complimentary chapters relat- tion are included. ing to the biological aspects of soils. The kinds The original Chapter 1, "Concepts of Soil," was and nature of soil organisms and nutrient cycling split into two chapters. Each chapter emphasizes remain as the central themes of Chapter 8. An an important concept of soil-soil as a medium expanded section on the rhizosphere has been for plant growth and soil as a natural body. Topics included. The distinctions between labile and sta- covered in Chapter 1 include the factors affecting ble organic matter and the interaction of organic plant growth, root growth and distribution, nutri- matter with the minerals (especially clays) are ent availability (including the roles of root inter- central themes of Chapter 9. Also, the concept of ception, mass flow and diffusion), and soil fertil- cation exchange capacity is minimally developed ity and productivity. The importance of soils as a in the coverage of the nature of soil organic matter source of nutrients and water is stressed in Chap- in Chapter 9. ter 1 and elsewhere throughout the book. Chapter Chapter 10, "Soil Mineralogy," and Chapter 11, 2 covers the basic soil formation processes of "Soil Chemistry", are complimentary chapters re- humification of organic matter, mineral weather- lating to the mineralogical and chemical proper- ing, leaching, and translocation of colloids. The ties of soils. The evolution theme included in important theme is soil as a three-dimensional Chapter 2 is used to develop the concept of body that is dynamic and ever-changing. The con- changing mineralogical and chemical properties cepts developed in the first two chapters are used with time. Soils are characterized as being mini- repeatedly throughout the book. mally, moderately, and intensively weathered, The next five chapters relate to soil physical and these distinctions are used in discussions of properties and water. The material on tillage and soil pH, liming, soil fertility and fertilizer use, soil traffic was expanded to reflect the increasing ef- genesis, and land use. fect of tillage and traffic on soils and plant growth Chapters 12 through 15 are concerned with the and is considered in Chapter 4. The nature of soil general area of soil fertility and fertilizer use. water is presented as a continuum of soil water Chapters 12 and 13 cover the macronutrients and potentials in Chapter 5. Darcy's law is developed micronutrients plus toxic elements, respectively.

V vi PREFACE

Chapters 14 and 15 cover the nature of fertilizers taxonomy is covered in Chapter 17. This allows a and the evaluation of soil fertility and the use of consideration of soil classification after soil fertilizers, respectively. Greater stress has been properties have been covered. This arrangement placed on mass flow and diffusion in regard to also makes the book more desirable for use in nutrient uptake. The interaction of water and soil two-year agricultural technology programs and fertility is developed, and there is expanded cov- overseas, in countries where Soil Taxonomy is not erage of soil fertility evaluation and the methods used. used to formulate fertilizer recommendations. The final chapter, "Land and the World Food Recognition is made of the increasing frequency Supply," includes a section on the world grain of high soil test results and the implications for trade and examines the importance of nonagro- fertilizer use and environmental quality. Greater nomic factors in the food-population problem. coverage is given to animal manure as both a Both English and metric units are used in the source of nutrients and a source of energy. Infor- measurement of crop yields, and for some other mation on land application of sewage sludge and parameters. Using both kinds of units should sat- on sustainable agriculture has been added. isfy both United States and foreign readers. Throughout these four chapters there is a greater Special thanks to Mary Foth for the artwork and emphasis on the importance of soil fertility and to my late son-in-law, Nate Rufe, for photographic fertilizers and on the environmental aspects of contributions. Over the years, many colleagues growing crops. have responded to my queries to expand my The next four chapters (Chapters 16, 17, 18, and knowledge and understanding. Others have pro- 19) relate to the areas of soil genesis, soil taxon- vided photographs. The reviewers also have pro- omy, soil geography and land use, and soil survey vided an invaluable service. To these persons, I and land use interpretations. In this edition, the am grateful. subjects of soil taxonomy (classification) and of Finally, this book is a STORY about soil. The soil survey and land use interpretations have re- story reflects my love of the soil and my devotion ceived increased coverage in two small chapters. to promoting the learning and understanding of The emphasis in the soil geography and land use soils for more than 40 years. I hope that all who chapter is at the suborder level. References to read this book will find it interesting as well as lower categories are few. Color photographs of informative. soil profiles are shown in Color Plates 5 and 6. No Henry D. Foth reference to Soil Taxonomy (USDA) is made until East Lansing, Michigan BRIEF CONTENTS

VII DETAILED CONTENTS

CHAPTER 1 CHAPTER 3 SOIL AS A MEDIUM FOR SOIL PHYSICAL PROPERTIES 22 PLANT GROWTH 1 SOIL TEXTURE 22 FACTORS OF PLANT GROWTH 1 The Soil Separates 22 Particle Size Analysis 24 Support for Plants 1 Soil Textural Classes 25 Essential Nutrient Elements 2 Determining Texture by the Field Method 25 Water Requirement of Plants 3 Influence of Coarse Fregments on Oxygen Requirement of Plants 4 Class Names 26 Freedom from Inhibitory Factors 5 Texture and the Use of Soils 26 PLANT ROOTS AND SOIL RELATIONS 5 SOIL STRUCTURE 27 Development of Roots in Soils 5 I mportance of Structure 28 Extensiveness of Roots in Soils 7 Genesis and Types of Structure 28 Extent of Root and Soil Contact 8 Grade and Class 29 Roles of Root Interception, Mass Flow, and Diffusion 8 Managing Soil Structure 29

SOIL FERTILITY AND SOIL PRODUCTIVITY 9 SOIL CONSISTENCE 31 Soil Consistence Terms 31

DENSITY AND WEIGHT RELATIONSHIPS 32 Particle Density and Bulk Density 32 Weight of a Furrow-Slice of Soil 33 Soil Weight on a Hectare Basis 34

CHAPTER 2 SOIL PORE SPACE AND POROSITY 34 SOIL AS A NATURAL BODY 11 Determination of Porosity 34 Effects of Texture and Structure on Porosity 35 THE PARENT MATERIAL OF SOIL 12 Porosity and Soil Aeration 35 Bedrock Weathering and Formation of Parent Material 12 SOIL COLOR 36 Sediment Parent Materials 13 Determination of Soil Color 37 Factors Affecting Soil Color 37 SOIL FORMATION 13 Significance of Soil Color 37 Soil-Forming Processes 14 Formation of A and C Hori zons 14 SOIL TEMPERATURE 38 Formation of B Horizons 14 Heat Balance of Soils 38 The Bt Horizon 15 Location and Temperature 39 The Bhs Horizon 17 Control of Soil Temperature 39 Permafrost 40 Formation of E Horizons 17 Formation of 0 Horizons 18 CHAPTER 4 SOILS AS NATURAL BODIES 18 TILLAGE AND TRAFFIC 42 The Soil-Forming Factors 18 Soil Bodies as Parts of Landscapes 19 EFFECTS OF TILLAGE ON SOILS AND How Scientists Study Soils as Natural Bodies 19 PLANT GROWTH 42 I mportance of Concept of Soil as Natural Body 20 Management of Crop Residues 42

X DETAILED CONTENTS

Tillage and Weed Control 43 CHAPTER 6 Effects of Tillage on Structure and Porosity 43 SOIL WATER MANAGEMENT 73 Surface Soil Crusts 44 Minimum and Zero Tillage Concepts 44 WATER CONSERVATION 73 Tilth and Tillage 45 Modifying the Infiltration Rate 73 Summer Fallowing 75 TRAFFIC AND SOIL COMPACTION 46 Saline Seep Due to Fallowing 76 Compaction Layers 46 Effect of Fertilizers on Water Use Efficiency 77 Effects of Wheel Traffic on Soils and Crops 47 Effects of Recreational Traffic 47 SOIL DRAINAGE 78 Effects of Logging Traffic on Soils and Water Table Depth Versus Air and Water Content Tree Growth 48 of Soil 79 Controlled Traffic 49 Benefits of Drainage 80 Surface Drainage 80 FLOODING AND PUDDLING OF SOIL 50 Subsurface Drainage 80 Effects of Flooding 50 Drainage in Soil of Container-Grown Plants 81 Effects of Puddling 50 Oxygen Relationships in Flooded Soils 51 IRRIGATION 82 Water Sources 82 Important Properties of Irrigated Soils 82 Water Application Methods 83 ,/ Flood Irrigation 83 CHAPTER 5 Furrow Irrigation 83 SOIL WATER 54 Sprinkler Irrigation 83 SOIL WATER ENERGY CONTINUUM 54 Subsurface Irrigation 85 Drip Irrigation Adhesion Water 55 85 Cohesion Water 55 Rate and Timing of Irrigation 85 Gravitational Water 56 Water Quality 86 Summary Statements 56 Total Salt Concentration 86 Sodium Adsorption Ratio 86 ENERGY AND PRESSURE RELATIONSHIPS 57 Boron Concentration 87 Pressure Relationships in Saturated Soil 57 Bicarbonate Concentration 87 Pressure Relationships in Unsaturated Soil 58 Salt Accumulation and Plant Response 89 THE SOIL WATER POTENTIAL 59 Salinity Control and Leaching Requirement 89 The Gravitational Potential 59 Effect of Irrigation on River Water Quality 93 The Matric Potential 59 Nature and Management of Saline and The Osmotic Potential 60 Sodic Soils 93 Measurement and Expression of Saline Soils 93 Water Potentials 60 Sodic Soils 93 Saline-Sodic Soils 94 SOIL WATER MOVEMENT 61 Water Movement in Saturated Soil 62 WASTEWATER DISPOSAL 94 Water Movement in Unsaturated Soil 63 Disposal of Septic Tank Effluent 94 Water Movement in Stratified Soil 63 Land Disposal of Municipal Wastewater 96 Water Vapor Movement 66 PRESCRIPTION ATHLETIC TURF 97 PLANT AND SOIL WATER RELATIONS 66 Available Water-Supplying Power of Soils 66 Water Uptake from Soils by Roots 67 Diurnal Pattern of Water Uptake 68 CHAPTER 7 Pattern of Water Removal from Soil 69 SOIL EROSION 100 Soil Water Potential Versus Plant Growth 69 Role of Water Uptake for Nutrient Uptake 71 WATER EROSION 100 Predicting Erosion Rates on Agricultural Land 100 SOIL WATER REGIME 71 R = The Rainfall Factor 101

DETAILED CONTENTS XI

K = The Soil Erodibility Factor 102 Mycorrhiza 126 LS = The Slope Length and Slope Nitrogen Fixation 127 Gradient Factors 103 C = The Cropping-Management Factor 104 SOIL ORGANISMS AND ENVIRONMENTAL P = The Erosion Control Practice Factor 105 QUALITY 128 Pesticide Degradation 128 Application of the Soil-Loss Equation 106 The Soil Loss Tolerance Value 107 Oil and Natural Gas Decontamination 128 Water Erosion on Urban Lands 108 EARTH MOVING BY SOIL ANIMALS 130 Water Erosion Costs 109 Earthworm Activity 130 Ants and Termites 130 WIND EROSION 110 Rodents 131 Types of Wind Erosion 110 Wind Erosion Equation 111 Factors Affecting Wind Erosion 111 Deep Plowing for Wind Erosion Control 113 Wind Erosion Control on Organic Soils 113 CHAPTER 9 SOIL ORGANIC MATTER 133

THE ORGANIC MATTER IN ECOSYSTEMS 133 CHAPTER 8 SOIL ECOLOGY 115 DECOMPOSITION AND ACCUMULATION 133 Decomposition of Plant Residues 134 THE ECOSYSTEM 115 Labile Soil Organic Matter 134 Producers 115 Stable Soil Organic Matter 135 Consumers and Decomposers 116 Decomposition Rates 136 Properties of Stable Soil Organic Matter 136 MICROBIAL DECOMPOSERS 116 Protection of Organic Matter by Clay 137 General Features of Decomposers 116 Bacteria 117 ORGANIC SOILS 139 Fungi 117 Organic Soil Materials Defined 139 Actinomycetes 118 Formation of Organic Soils 139 Vertical Distribution of Decomposers in Properties and Use 140 the Soil 119 Archaeological Interest 140 SOIL ANIMALS 119 THE EQUILIBRIUM CONCEPT 141 Worms 120 A Case Example 141 Earthworms 120 Effects of Cultivation 142 Nematodes 121 Maintenance of Organic Matter in Arthropods 121 Cultivated Fields 143 Springtails 121 Effects of Green Manure 144 Mites 122 HORTICULTURAL USE OF ORGANIC MATTER 144 Millipedes and Centipedes 122 Horticultural Peats 145 White Grubs 122 Composts 145 Interdependence of Microbes and Animals in Decomposition 123 NUTRIENT CYCLING 123 Nutrient Cycling Processes 123 A Case Study of Nutrient Cycling 124 CHAPTER 10 Effect of Crop Harvesting on Nutrient Cycling 124 SOIL MINERALOGY 148 SOIL MICROBE AND ORGANISM CHEMICAL AND MINERALOGICAL COMPOSITION OF INTERACTIONS 125 THE EARTH'S CRUST 148 The Rhizosphere 125 Chemical Composition of the Earth's Crust 148 Disease 126 Mineralogical Composition of Rocks 149

DETAILED CONTENTS

WEATHERING AND SOIL MINERALOGICAL MANAGEMENT OF SOIL pH 178 COMPOSITION 149 Lime Requirement 179 Weathering Processes 150 Lime Requirement of Intensively Summary Statement 150 Weathered Soils 180 Weathering Rate and Crystal Structure 151 Lime Requirement of Minimally and Moderately Mineralogical Composition Versus Soil Age 153 Weathered Soils 180 Summary Statement 155 The Liming Equation and Soil Buffering 181 SOIL CLAY MINERALS 155 Some Considerations in Lime Use 182 Mica and Vermiculite 156 Management of Calcareous Soils 182 Smectites 158 Soil Acidulation 183 Kaolinite 159 EFFECTS OF FLOODING ON CHEMICAL Allophane and Halloysite 160 PROPERTIES 183 Oxidic Clays 160 Dominant Oxidation and Reduction Reactions 183 Summary Statement 161 Effect on Soil pH 184 ION EXCHANGE SYSTEMS OF SOIL CLAYS 161 Layer Silicate System 161 CHAPTER 12 Oxidic System 162 PLANT-SOIL MACRONUTRIENT Oxide-Coated Layer Silicate System 162 RELATIONS 186

DEFICIENCY SYMPTOMS 186 CHAPTER 11 NITROGEN 186 SOIL CHEMISTRY 164 The Soil Nitrogen Cycle 187 Dinitrogen Fixation 187 CHEMICAL COMPOSITION OF SOILS 164 Symbiotic Legume Fixation 187 ION EXCHANGE 165 Nonlegume Symbiotic Fixation 192 Nature of Cation Exchange 165 Nonsymbiotic Nitrogen Fixation 192 Cation Exchange Capacity of Soils 166 Summary Statement 192 Cation Exchange Capacity Versus Soil pH 167 Mineralization 192 Kinds and Amounts of Exchangeable Cations 168 Nitrification 193 Exchangeable Cations as a Source of Plant Immobilization 194 Nutrients 169 Carbon-Nitrogen Relationships 195 Anion Exchange 169 Denitrification 195 SOIL pH 170 Human Intrusion in the Nitrogen Cycle 196 Determination of Soil pH 170 Summary Statement on Nitrogen Cycle 197 Sources of Alkalinity 170 Plant Nitrogen Relations 197 Carbonate Hydrolysis 170 PHOSPHORUS 197 Mineral Weathering 171 Soil Phosphorus Cycle 198 Sources of Acidity 171 Effect of pH on Phosphorus Availability 199 Development and Properties of Acid Soils 171 Changes in Soil Phosphorus Over Time 199 Role of Aluminum 172 Plant Uptake of Soil Phosphorus 200 Moderately Versus Intensively Plant Phosphorus Relations 201 Weathered Soils 173 POTASSIUM 202 174 Role of Strong Acids Soil Potassium Cycle 202 Acid Rain Effects 174 Summary Statement 203 Soil Buffer Capacity 174 Plant Uptake of Soil Potassium 204 176 Summary Statement Plant Potassium Relations 205 SIGNIFICANCE OF SOIL pH 176 CALCIUM AND MAGNESIUM 205 Nutrient Availability and pH 177 Plant Calcium and Magnesium Relations 206 178 Effect of pH on Soil Organisms Soil Magnesium and Grass Tetany 206 Toxicities in Acid Soils 178 pH Preferences of Plants 178 SULFUR 207

xiii DETAILED CONTENTS

CHAPTER 13 APPLICATION AND USE OF FERTILIZERS 237 MICRONUTRIENTS AND Time of Application 238 Methods of Fertilizer Placement 238 TOXIC ELEMENTS 210 Salinity and Acidity Effects 240 IRON AND MANGANESE 210 ANIMAL MANURES 241 Plant "Strategies" for Iron Uptake 211 Manure Composition and Nutrient Value 241 Nitrogen Volatilization Loss from Manure 242 COPPER AND ZINC 212 Plant Copper and Zinc Relations 213 Manure as a Source of Energy 243 BORON 214 LAND APPLICATION OF SEWAGE SLUDGE 244 Sludge as a Nutrient Source 244 CHLORINE 214 Heavy Metal Contamination 245 MOLYBDENUM 214 FERTILIZER USE AND ENVIRONMENTAL Plant and Animal Molybdenum Relations 215 QUALITY 246 Phosphate Pollution 246 COBALT 216 Nitrate Pollution 246 SELENIUM 217 Nitrate Toxicity 247 POTENTIALLY TOXIC ELEMENTS SUSTAINABLE AGRICULTURE 247 FROM POLLUTION 217

RADIOACTIVE ELEMENTS 218 CHAPTER 16 SOIL GENESIS 250

CHAPTER 14 ROLE OF TIME IN SOIL GENESIS 250 FERTILIZERS 221 Case Study of Soil Genesis 250 Time and Soil Development Sequences 252 221 FERTILIZER TERMINOLOGY ROLE OF PARENT MATERIAL IN SOIL GENESIS 253 Grade and Ratio 221 Consolidated Rock as a Source of General Nature of Fertilizer Laws 222 Parent Material 253 Types of Fertilizers 222 Soil Formation from Limestone Weathering 253 FERTILIZER MATERIALS 222 Sediments as a Source of Parent Material 254 Nitrogen Materials 222 Gulf and Atlantic Coastal Plains 255 Phosphorus Materials 224 Central Lowlands 256 Potassium Materials 226 Interior Plains 258 Micronutrient Materials 228 Basin and Range Region 258 Volcanic Ash Sediments 258 MIXED FERTILIZERS 228 Effect of Parent Material Properties on Granular Fertilizers 228 Soil Genesis 258 Bulk Blended Fertilizers 229 Stratified Parent Materials 259 Fluid Fertilizers 229 Parent Material of Organic Soils 260 NATURAL FERTILIZER MATERIALS 230 ROLE OF CLIMATE IN SOIL GENESIS 260 Precipitation Effects 260 Temperature Effects 262 CHAPTER 15 Climate Change and Soil Properties 263 SOIL FERTILITY EVALUATION AND ROLE OF ORGANISMS IN SOIL GENESIS 263 FERTILIZER USE 232 Trees Versus Grass and Organic Matter Content 263 SOIL FERTILITY EVALUATION 232 Vegetation Effects on Leaching and Eluviation 264 Plant Deficiency Symptoms 232 Role of Animals in Soil Genesis 265 Plant Tissue Tests 232 Soil Tests 233 ROLE OF TOPOGRAPHY IN SOIL GENESIS 265 Computerized Fertilizer Recommendations 235 Effect of Slope 265 xiv DETAILED CONTENTS

Effects of Water Tables and Drainage 266 ANDISOLS 294 Topography, Parent Material, and Genesis and Properties 294 Time Interactions 267 Suborders 294 Uniqueness of Soils Developed in Alluvial ARIDISOLS 294 267 Parent Material Genesis and Properties 295 HUMAN BEINGS AS A SOIL-FORMING FACTOR 269 Aridisol Suborders 295 Land Use on Aridisols 296

CHAPTER 17 ENTISOLS 297 Aquents 297 SOIL TAXONOMY 271 Fluvents 297 DIAGNOSTIC SURFACE HORIZONS 271 Psamments 298 Mollic Horizon 271 Orthents 298 Umbric and Ochric Horizons 272 HISTOSOLS 299 Histic Horizon 272 Histosol Suborders 299 Melanic Horizon 272 Land Use on Histosols 300 Anthropic and Plaggen Horizons 273 INCEPTISOLS 300 DIAGNOSTIC SUBSURFACE HORIZONS 273 Aquepts 300 Cambic Horizon 273 Ochrepts 301 Argillic and Natric Horizons 274 Umbrepts 301 Kandic Horizon 274 Spodic Horizon 275 MOLLISOLS 302 Albic Horizon 275 Aquolls 302 Oxic Horizon 275 Borolls 303 Calcic, Gypsic, and Salic Horizons 275 Ustolls and Udolls 303 Subordinate Distinctions of Horizons 276 Xerolls 304 SOIL MOISTURE REGIMES 276 OXISOLS 304 Aquic Moisture Regime 276 Oxisol Suborders 306 Udic and Perudic Moisture Regime 276 Land Use on Udox Soils 306 Ustic Moisture Regime 277 Land Use on Ustox Soils 307 Aridic Moisture Regime 277 Extremely Weathered Oxisols 307 Xeric Moisture Regime 278 Plinthite or Laterite 308 SOIL TEMPERATURE REGIMES 279 SPODOSOLS 308 Spodosol Suborders 308 CATEGORIES OF SOIL TAXONOMY 279 Spodosol Properties and Land Use 309 Soil Order 279 Suborder and Great Group 282 ULTISOLS 311 Subgroup, Family, and Series 283 Ultisol Suborders 311 Properties of Ultisols 311 AN EXAMPLE OF CLASSIFICATION: Land Use on Ultisols 312 THE ABAC SOILS 283 VERTISOLS 312 THE PEDON 284 Vertisol Suborders 313 Vertisol Genesis 313 Vertisol Properties 314 CHAPTER 18 Land Use on Vertisols 315 SOIL GEOGRAPHY AND LAND USE 285

ALFISOLS 285 CHAPTER 19 Aqualfs 285 SOIL SURVEYS AND LAND-USE Boralfs 286 INTERPRETATIONS 318 Udalfs 286 Ustalfs 293 MAKING A SOIL SURVEY 318 Xeralfs 293 Making a Soil Map 318 DETAILED CONTENTS XV

Writing the Soil Survey Report 320 FUTURE OUTLOOK 332 Using the Soil Survey Report 321 Beyond Technology 333 The World Grain Trade 333 SOIL SURVEY INTERPRETATIONS AND LAND-USE PLANNING 322 Population Control and Politics 334 Examples of Interpretative Land-Use Maps 322 Land Capability Class Maps 323 Computers and Soil Survey Interpretations 323 APPENDIX I SOIL SURVEYS AND AGROTECHNOLOGY SOIL TEXTURE BY THE TRANSFER 324 FIELD METHOD 337

CHAPTER 20 APPENDIX II LAND AND THE WORLD TYPES AND CLASSES OF FOOD SUPPLY 326 SOIL STRUCTURE 339 POPULATION AND FOOD TRENDS 326 Development of Agriculture 326 The Industrial Revolution 326 APPENDIX III Recent Trends in Food Production 327 PREFIXES AND THEIR CONNOTATIONS Recent Trends in Per Capita Cropland 328 Summary Statement 329 FOR NAMES OF GREAT GROUPS IN THE U.S. SOIL CLASSIFICATION SYSTEM POTENTIALLY AVAILABLE LAND AND (SOIL TAXONOMY) 341 SOIL RESOURCES 329 World's Potential Arable Land 329 Limitations of World Soil Resources 332 GLOSSARY 342 Summary Statement 332 INDEX 353 CHAPTER 1

SOIL AS A MEDIUM FOR PLANT GROWTH

SOIL. Can you think of a substance that has had serve as channels for the movement of air and more meaning for humanity? The close bond that water. Pore spaces are used as runways for small ancient civilizations had with the soil was ex- animals and are avenues for the extension and pressed by the writer of Genesis in these words: growth of roots. Roots anchored in soil suppport plants and roots absorb water and nutrients. For And he Lod God fomed Man of d fom he good plant growth, the root-soil environment gond. should be free of inhibitory factors. The three There has been, and is, a reverence for the ground essential things that plants absorb from the soil or soil. Someone has said that "the fabric of hu- and use are: (1) water that is mainly evaporated man life is woven on earthen looms; everywhere it from plant leaves, (2) nutrients for nutrition, and smells of clay." Even today, most of the world's (3) oxygen for root respiration. people are tillers of the soil and use simple tools to produce their food and fiber. Thus, the concept of soil as a medium of plant growth was born in Support for Plants antiquity and remains as one of the most impor- One of the most obvious functions of soil is to tant concepts of soil today (see Figure 1.1). provide support for plants. Roots anchored in soil enable growing plants to remain upright. Plants grown by hydroponics (in liquid nutrient culture) are commonly supported on a wire framework. FACTORS OF PLANT GROWTH Plants growing in water are supported by the The soil can be viewed as a mixture of mineral buoyancy of the water. Some very sandy soils that and organic particles of varying size and composi- are droughty and infertile provide plants with little tion in regard to plant growth. The particles oc- else than support. Such soils, however, produce cupy about 50 percent of the soil's volume. The high-yielding crops when fertilized and frequently remaining soil volume, about 50 percent, is pore irrigated. There are soils in which the impenetra- space, composed of pores of varying shapes and ble nature of the subsoil, or the presence of water- sizes. The pore spaces contain air and water and saturated soil close to the soil surface, cause shal-

1 z SOIL AS A MEDIUM FOR PLANT GROWTH

FIGURE 1.1 Wheat harvest near the India-Nepal border. About one half of the world's people are farmers who are closely tied to the land and make their living producing crops with simple tools.

low rooting. Shallow-rooted trees are easily blown some plants. Plants deficient in an essential ele- over by wind, resulting in indho. ment tend to exhibit symptoms that are unique for that element, as shown in Figure 1.2. More than 40 other elements have been found Essential Nutrient Elements Plants need certain essential nutrient elements to FIGURE 1.2 Manganese deficiency symptoms on complete their life cycle. No other element can kidney beans. The youngest, or upper leaves, have completely substitute for these elements. At least light-green or yellow-colored intervein areas and 16 elements are currently considered essential for dark-green veins. the growth of most vascular plants. Carbon, hydrogen, and oxygen are combined in photosynthetic reactions and are obtained from air and water. These three elements compose 90 percent or more of the dry matter of plants. The remaining 13 elements are obtained largely from the soil. Nitrogen (N), phosphorus (P), potassium (K), calcium Ca), magnesium (Mg), and sulfur (S) are required in relatively large amounts and are referred to as the macronutrients. Elements required in considerably smaller amount are called the micronutrients. They include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). Cobalt (Co) is a micronutrient that is needed by only

FACTORS OF PLANT GROWTH 3

in plants. Some plants accumulate elements that Cations are positively charged ions such as Ca t are not essential but increase growth or quality. and K + and anions are negatively charged ions - The absorption of sodium (Na) by celery is an such as NO3- (nitrate) and H 2 PO4 (phosphate). example, and results in an improvement of flavor. The amount of cations absorbed by a plant is Sodium can also be a substitute for part of the about chemically equal to the amount of anions potassium requirement of some plants, if po- absorbed. Excess uptake of cations, however, re- tassium is in low supply. Silicon (Si) uptake may sults in excretion of H+ and excess uptake of - - increase stem strength, disease resistance, and anions results in excretion of OH or HCO 3 to growth in grasses. maintain electrical neutrality in roots and soil. Most of the nutrients in soils exist in the miner- The essential elements that are commonly ab- als and organic matter. Minerals are inorganic sorbed from soils by roots, together with their substances occurring naturally in the earth. They chemical symbols and the uptake forms, are listed have a consistent and distinctive set of physical in Table 1.1. properties and a chemical composition that can In nature, plants accommodate themselves to be expressed by a formula. Quartz, a mineral the supply of available nutrients. Seldom or rarely composed of SiO2, is the principal constituent of is a soil capable of supplying enough of the essen- ordinary sand. Calcite (CaCO 3) is the primary tial nutrients to produce high crop yields for any mineral in limestone and chalk and is abundant is reasonable period of time after natural or virgin many soils. Orthclase-feldspar (KAISi3 O8 ) is a lands are converted to cropland. Thus, the use of very common soil mineral, which contains po- animal manures and other amendments to in- tassium. Many other minerals exist in soils be- crease soil fertility (increase the amount of nutri- cause soils are derived from rocks or materials ent ions) are ancient soil management practices. containing a wide variety of minerals. Weathering of minerals brings about their decomposition and the production of ions that are released into the soil water. Since silicon is not an essential ele- Water Requirement of Plants ment, the weathering of quartz does not supply an A few hundred to a few thousand grams of water essential nutrient, plants do not depend on these are required to produce 1 gram of dry plant mate- minerals for their oxygen. The weathering of cal- rial. Approximately one percent of this water be- cite supplies calcium, as Cat+ , and the weather- comes an integral part of the plant. The remainder ing of orthoclase releases potassium as K+. of the water is lost through transpiration, the loss The organic matter in soils consists of the re- of water by evaporation from leaves. Atmospheric cent remains of plants, microbes, and animals conditions, such as relative humidity and temper- and the resistant organic compounds resulting ature, play a major role in determining how from the rotting or decomposition processes. De- quickly water is transpired. composition of soil organic matter releases es- The growth of most economic crops will be sential nutrient ions into the soil water where the curtailed when a shortage of water occurs, even ions are available for another cycle of plant though it may be temporary. Therefore, the soil's growth. ability to hold water over time against gravity is Available elements or nutrients are those nutri- important unless rainfall or irrigation is adequate. ent ions or compounds that plants and microor- Conversely, when soils become water saturated, ganisms can absorb and utilize in their growth. the water excludes air from the pore spaces and Nutrients are generally absorbed by roots as creates an oxygen deficiency. The need for the cations and anions from the water in soils, or the removal of excess water from soils is related to the soil solution. The ions are electrically charged. need for oxygen. 4 SOIL AS A MEDIUM FOR PLANT GROWTH

TABLE 1.1 Chemical Symbols and Common Forms Oxygen Requirement of Plants of the Essential Elements Absorbed by Plant Roots Roots have openings that permit gas exchange. from Soils Oxygen from the atmosphere diffuses into the soil and is used by root cells for respiration. The carbon dioxide produced by the respiration of roots, and microbes, diffuses through the soil pore space and exits into the atmosphere. Respiration releases energy that plant cells need for synthesis and translocation of the organic compounds needed for growth. Frequently, the concentration of nutrient ions in the soil solution is less than that in roots cells. As a consequence, respiration energy is also used for the active accumulation of nutrient ions against a concentration gradient. Some plants, such as water lilies and rice, can grow in water-saturated soil because they have morphological structures that permit the diffusion of atmospheric oxygen down to the roots. Suc- cessful production of most plants in water culture requires aeration of the solution. Aerobic micro-

organisms require molecular oxygen (O 2) and use oxygen from the soil atmosphere to decompose

FIGURE 1.3 The soil in which these tomato plants organic matter and convert unavailable nutrients were growing was saturated with water. The stopper in organic matter into ionic forms that plants can at the bottom of the left container was immediately reuse (nutrient cycling). removed, and excess water quickly drained away. Great differences exist between plants in their The soil in the right container remained water saturated and the plant became severely wilted ability to tolerate low oxygen levels in soils. Sensi- within 24 hours because of an oxygen deficiency.

FIGURE 1.4 Soil salinity (soluble salt) has seriously affected the growth of sugar beets in the foreground of this irrigated field.

PLANT ROOTS AND SOIL RELATIONS 5

tive plants may be wilted and/or killed as a result etc.) in the shoot via photosynthesis and translo- of saturating the soil with water for a few hours, as cation of food downward for root growth, and shown in Figure 1.3. The wilting is believed to (2) the absorption of water and nutrients by roots result from a decrease in the permeability of the and the upward translocation of water and nutri- roots to water, which is a result of a disturbance of ents to the shoot for growth. metabolic processes due to an oxygen deficiency. After a root emerges from the seed, the root tip elongates by the division and elongation of cells in the meristematic region of the root cap. After Feed f Ihibi Fac the root cap invades the soil, it continues to elon- Abundant plant growth requires a soil environ- gate and permeate the soil by the continued divi- ment that is free of inhibitory factors such as toxic sion and elongation of cells. The passage of the substances, disease organisms, impenetrable root tip through the soil leaves behind sections of layers, extremes in temperature and acidity or root that mature and become "permanent" resi- basicity, or an excessive salt content, as shown in dents of the soil. Figure 1.4. As the plant continues to grow and roots elongate throughout the topsoil, root extension into the subsoil is likely to occur. The subsoil environment will be different in terms of the supply of PLANT ROOTS AND SOIL RELATIONS water, nutrients, oxygen, and in other growth fac- Plants utilize the plant growth factors in the soil by tors. This causes roots at different locations in the way of the roots. The density and distribution of soil (topsoil versus subsoil) to perform different roots affect the amount of nutrients and water that functions or the same functions to varying de- roots extract from soils. Perennials, such as oak grees. For example, most of the nitrogen will and alfalfa, do not reestablish a completely new probably be absorbed by roots from the topsoil root system each year, which gives them a distinct because most of the organic matter is concen- advantage over annuals such as cotton or wheat. trated there, and nitrate-nitrogen becomes avail- Root growth is also influenced by the soil environ- able by the decomposition of organic matter. By ment; consequently, root distribution and density contrast, in soils with acid topsoils and alkaline are a function of both the kind of plant and the subsoils, deeply penetrating roots encounter a nature of the root environment. great abundance of calcium in the subsoil. Under these conditions, roots in an alkaline subsoil may absorb more calcium than roots in an acid top- Deee f R i Si soil. The topsoil frequently becomes depleted of A seed is a dormant plant. When placed in moist, water in dry periods, whereas an abundance of warm soil, the seed absorbs water by osmosis and water still exists in the subsoil. This results in a swells. Enzymes activate, and food reserves in the relatively greater dependence on the subsoil for endosperm move to the embryo to be used in water and nutrients. Subsequent rains that rewet germination. As food reserves are exhausted, the topsoil cause a shift to greater dependence on green leaves develop and photosynthesis begins. the topsoil for water and nutrients. Thus, the man- The plant now is totally dependent on the sun for ner in which plants grow is complex and changes energy and on the soil and atmosphere for nutri- continually throughout the growing season. In ents and water. In a sense, this is a critical period this regard, the plant may be defined as an inte- in the life of a plant because the root system is grator of a comple and ever changing set of small. Continued development of the plant re- environmental conditions. quires: (1) the production of food (carbohydrates, The root systems of some common agricultural SOIL AS A MEDIUM FOR PLANT GROWTH

FIGURE 1.6 Four stages for the development of the shoots and roots of corn (Zea maize). Stage one (left) shows dominant downward and diagonal root growth, stage two shows "filling" of the upper soil layer with roots, stage three shows rapid elongation of stem and deep root growth, and stage four (right) shows development of the ears (grain) and brace root growth. FIGURE 1.5 The tap root systems of two-week old soybean plants. Note the many fine roots branching off the tap roots and ramifying soil. (Scale on the systems and shoots during the growing season right is in inches.) revealed a synchronization between root and shoot growth. Four major stages of development crops were sampled by using a metal frame to were found. Corn has a fibrous root system, and collect a 10-centimeter-thick slab of soil from the early root growth is mainly by development of wall of a pit. The soil slab was cut into small roots from the lower stem in a downward and blocks and the roots were separated from the soil diagonal direction away from the base of the plant using a stream of running water. Soybean plants (stage one of Figure 1.6). The second stage of root were found to have a tap root that grows directly growth occurs when most of the leaves are de- downward after germination, as shown in Figure veloping and lateral roots appear and "fill" or 1.5. The tap roots of the young soybean plants are space themselves uniformily in the upper 30 to 40 several times longer than the tops or shoots. Lat- centimeters of soil. Stage three is characterized by eral roots develop along the tap roots and space rapid elongation of the stem and extension of themselves uniformily throughout the soil occu- roots to depths of 1 to 2 meters in the soil. Finally, pied by roots. At maturity, soybean taproots will during stage four, there is the production of the extend about 1 meter deep in permeable soils ear or the grain. Then, brace roots develop from with roots well distributed throughout the topsoil the lower nodes of the stem to provide anchorage and subsoil. Alfalfa plants also have tap roots that of the plant so that they are not blown over by the commonly penetrate 2 to 3 meters deep; some wind. Brace roots branch profusely upon entering have been known to reach a depth of 7 meters. the soil and also function for water and nutrient Periodic sampling of corn (Zea maize) root uptake. PLANT ROOTS AND SOIL RELATIONS 7

There is considerable uniformity in the lateral distribution of roots in the root zone of many crops (see Figure 1.7). This is explained on the basis of two factors. First, there is a random distribution of pore spaces that are large enough for root extension, because of soil cracks, channels formed by earthworms, or channels left from previous root growth. Second, as roots elongate through soil, they remove water and nutrients, which makes soil adjacent to roots a less favorable environment for future root growth. Then, roots grow preferentially in areas of the soil devoid of roots and where the supply of water and nutrients is more favorable for root growth. This results in a fairly uniform distribution of roots throughout the root zone unless there is some barrier to root extension or roots encounter an unfavorable environment. Most plant roots do not invade soil that is dry, nutrient deficient, extremely acid, or water saturated and lacking oxygen. The preferential development of yellow birch roots in loam soil, compared with sand soil, because of a more favorable

FIGURE 1.8 Development of the root system of a yellow birch seedling in sand and loam soil. Both soils had adequate supplies of water and oxygen, but, the loam soil was much more fertile. (After Redmond, 1954.)

FIGURE 1.7 Roots of mature oat plants grown in rows 7 inches (18 cm) apart. The roots made up 13 percent of the total plant weight. Note the uniform, lateral, distribution of roots between 3 and 24 inches (8 and 60 cm). Scale along the left is in inches.

Extensiveness of Roots in Soil Roots at plant maturity comprise about 10 percent of the entire mass of cereal plants, such as corn, wheat, and oats. The oat roots shown in Figure 1.7 weighed 1,767 pounds per acre (1,979 kg/ha) and made up 13 percent of the total plant weight. For trees, there is a relatively greater mass of roots as compared with tops, commonly in the range of 15 to 25 percent of the entire tree. SOIL AS A MEDIUM FOR PLANT GROWTH combination of nutrients and water, is shown in Roles of Root Interception, Mass Flow, Figure 1.8. and Diffusion Water and nutrients are absorbed at sites located on or at the surface of roots. Elongating roots Extent of Root and Soil Contact directly encounter or intercept water and nutrient A rye plant was grown in 1 cubic foot of soil for 4 ions, which appear at root surfaces in position for months at the University of Iowa by Dittmer (this absorption. This is root interception and accounts study is listed amoung the references at the end of for about 1 percent or less of the nutrients ab- the chapter). The root system was carefully re- sorbed. The amount intercepted is in proportion moved from the soil by using a stream of running to the very limited amount of direct root and soil water, and the roots were counted and measured contact. for size and length. The plant was found to have Continued absorption of water adjacent to the hundreds of kilometers (or miles) of roots. Based root creates a lower water content in the soil near on an assumed value for the surface area of the the root surface than in the soil a short distance soil, it was calculated that 1 percent or less of the away. This difference in the water content be- soil surface was in direct contact with roots. tween the two points creates a water content gra- Through much of the soil in the root zone, the dient, which causes water to move slowly in the distance between roots is approximately 1 cen- direction of the root. Any nutrient ions in the water timeter. Thus, it is necessary for water and nutri- are carried along by flow of the water to root ent ions to move a short distance to root surfaces surfaces where the water and nutrients are both in for the effective absorption of the water and nutri- position for absorption. Such movement of nutrients. The limited mobility of water and most of the ents is called mass flo. nutrients in moist and well-aerated soil means The greater the concentration of a nutrient in that only the soil that is invaded by roots can the soil solution, the greater is the quantity of the contribute significantly to the growth of plants. nutrient moved to roots by mass flow. The range

TABLE 1.2 Relation Between Concentration of Ions in the Soil Solution and Concentration within the Corn Plant

Adapted from S. A. Barber, "A Diffusion and Mass Flow Concept of Soil Nutrient Availability, "Soil Sci., 93:39-49, 1962. Used by permission of the author and The Williams and Wilkins Co., Baltimore. a Dry weight basis. SOIL FERTILITY AND SOIL PRODUCTIVITY 9 of concentration for some nutrients in soil water is nutrient also depends on the amount needed. Cal- given in Table 1.2. The calcium concentration cium is rarely deficient for plant growth, partially (Table 1.2) ranges from 8 to 450 parts per million because plants' needs are low. As a consequence, (ppm). For a concentration of only 8 ppm in soil these needs are usually amply satisfied by the water, and 2,200 ppm of calcium in the plant, the movement of calcium to roots by mass flow. The plant would have to absorb 275 (2,200/8) times same is generally true for magnesium and sulfur. more water than the plant's dry weight to move the The concentration of nitrogen in the soil solution calcium needed to the roots via mass flow. Stated tends to be higher than that for calcium, but be- in another way, if the transpiration ratio is 275 cause of the high plant demand for nitrogen, (grams of water absorbed divided by grams of about 20 percent of the nitrogen that plants ab- plant growth) and the concentration of calcium in sorb is moved to root surfaces by diffusion. Diffu- the soil solution is 8 ppm, enough calcium will be sion is the most important means by which phos- moved to root surfaces to supply the plant need. phorus and potassium are transported to root Because 8 ppm is a very low calcium concentra- surfaces, because of the combined effects of con- tion in soil solutions, and transpiration ratios are centration in the soil solution and plant demands. usually greater than 275, mass flow generally Mass flow can move a large amount of nutrients moves more calcium to root surfaces than plants rapidly, whereas diffusion moves a small amount need. In fact, calcium frequently accumulates of nutrients very slowly. Mass flow and diffusion along root surfaces because the amount moved to have a limited ability to move phosphorus and the roots is greater than the amount of calcium potassium to roots in order to satisfy the needs of that roots absorb. crops, and this limitation partly explains why a Other nutrients that tend to have a relatively large amount of phosphorus and potassium is large concentration in the soil solution, relative to added to soils in fertilizers. Conversely, the large the concentration in the plant, are nitrogen, mag- amounts of calcium and magnesium that are nesium, and sulfur (see Table 1.2). This means moved to root surfaces, relative to crop plant that mass flow moves large amount of these nutri- needs, account for the small amount of calcium ents to roots relative to plant needs. and magnesium that is added to soils in fertilizers. Generally, mass flow moves only a small amount of the phosphorus to plant roots. The phosphorus concentration in the soil solution is Summary Statement usually very low. For a soil solution concentration The available nutrients and available water are the of 0.03 ppm and 2,000 ppm plant concentration, nutrients and water that roots can absorb. The the transpiration ratio would need to be more than absorption of nutrients and water by roots is de- 60,000. This illustration and others that could be pendent on the surface area-density (cm 2 /cm 3 ) of drawn from the data in Table 1.2, indicate that roots. Mathematically: some other mechanism is needed to account for the movement of some nutrients to root surfaces. uptake = availability x surface area-density This mechanism or process is known as diffusion. Diffusion is the movement of nutrients in soil water that results from a concentration gradient. SOIL FERTILITY AND Diffusion of ions occurs whether or not water is SOIL PRODUCTIVITY moving. When an insufficient amount of nutrients is moved to the root surface via mass flow, diffu- Soil fertility is defined as he abili of a oil o sion plays an important role. Whether or not ppl eenial elemen fo plan goh ih- plants will be supplied a sufficient amount of a o a oic concenaion of an elemen. Soil 10 SOIL AS A MEDIUM FOR PLANT GROWTH fertility refers to only one aspect of plant growth- and distribution of roots in soils, and (3) the the adequacy, toxicity, and balance of plant nutri- movement of nutrients, water, and air to root sur- ents. An assessment of soil fertility can be made faces for absorption. Soils are productive in terms with a series of chemical tests. of their ability to produce plants. Soil productivity is the soil's capacity to pro- The concept of soil as a medium for plant duce a certain yield of crops or other plants with growth views the soil as a material of fairly uni- optimum management. For example, the produc- form composition. This is entirely satisfactory tivity of a soil for cotton production is commonly when plants are grown in containers that contain expressed as kilos, or bales of cotton per acre, or a soil mix. Plants found in fields and forests, hectare, when using an optimum management however, are growing in soils that are not uniform. system. The optimum managment system spec- Differences in the properties between topsoil and ifies such factors as planting date, fertilization, subsoil layers affect water and nutrient absorp- irrigation schedule, tillage, cropping sequence, tion. It is natural for soils in fields and forests to be and pest control. Soil scientists determine soil composed of horizontal layers that have different productivity ratings of soils for various crops by properties, so it is also important that agricultur- measuring yields (including tree growth or timber ists and foresters consider soils as natural bodies. production) over a period of time for those pro- This concept is also useful for persons involved in duction uses that are currently relevant. Included the building of engineering structures, solving en- in the measurement of soil productivity are the vironment problems such as nitrate pollution of influence of weather and the nature and aspect of groundwater, and using the soil for waste dis- slope, which greatly affects water runoff and ero- posal. The soil as a natural body is considered in sion. Thus, soil productivity is an expression of all the next chapter. the factors, soil and nonsoil, that influence crop yields. For a soil to produce high yields, it must be fertile for the crops grown. It does not follow, REFERENCES however, that a fertile soil will produce high Barber, S. A. 1962. "A Diffusion and Mass Flow Concept yields. High yields or high soil productivity de- of Soil Nutrient Availability." Soil Sci. 93:39-49. pends on optimum managment systems. Many Dittmer, H. J. 1937. "A Quantitative Study of the Roots fertile soils exist in arid regions but, within man- and Root Hairs of a Winter Rye Plant." Am. Jour. Bot. agement systems that do not include irrigation, 24:417-420. Foth, H. D. 1962. "Root and Top Growth of Corn." these soils are unproductive for corn or rice. Agron. Jour. 54:49-52. Foth, H. D., L. S. Robertson, and H. M. Brown. 1964. "Effect of Row Spacing Distance on Oat Perfor- mance." Agron. Jour. 56:70-73. SUMMARY Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. John Wiley, New York. The concept of soil as a medium for plant growth Redmond, D. R. 1954. "Variations in Development of is an ancient concept and dates back to at least Yellow Birch Roots in Two Soil Types." Forestry the beginning of agriculture. The concept empha- Chronicle. 30:401-406. sizes the soil's role in the growth of plants. Impor- Simonson, R. W. 1968. "Concept of Soil." Adv. in Agron. tant aspects of the soil as a medium for plant 20:1-47. Academic Press, New York. growth are: (1) the role of the soil in supplying Wadleigh, C. H. 1957. "Growth of Plants," in Soil, USDA plants with growth factors, (2) the development Yearbook of Agriculture. Washington, D.C. CHAPTER 2

SOIL AS A NATURAL BODY

One day a colleague asked me why the alfalfa clay content than the two upper layers. The roots plants on some research plots were growing so penetrated this layer with no difficulty, however. poorly. A pit was dug in the field and a vertical Below layer three, the alfalfa tap root encountered section of the soil was sampled by using a metal a layer (layer four) that was impenetrable (too frame. The sample of soil that was collected was 5 compact), with the root growing above it in a centimeters thick, 15 centimeters wide, and 75 lateral direction. From these observations it was centimeters long. The soil was glued to a board concluded that the alfalfa grew poorly because and a vacuum cleaner was used to remove loose the soil material below a depth of 58 centimeters: soil debris and expose the natural soil layers and (1) created a barrier to deep root penetration, roots. Careful inspection revealed four soil layers which resulted in a less than normal supply of as shown in Figure 2.1. water for plant growth during the summer, and The upper layer, 9 inches (22 cm) thick, is the (2) created a water-saturated zone above the third plow layer. It has a dark color and an organic layer that was deficient in oxygen during wet pe- matter content larger than any of the other layers. riods in the spring. The fact that the soil occurred Layer two, at the depth of 9 to 14 inches (22 to naturally in a field raises such questions as: What 35 cm) differs from layer one by having a light-gray kinds of layers do soils have naturally? How do the color and a lower organic matter content. Both layers form? What are their properties? How do layers are porous and permeable for the move- these layers affect how soils are used? The an- ment of air and water and the elongation of roots. swers to these questions require an understand- In layer three, at a depth of 14 to 23 inches (35 to ing that landscapes consist of three-dimensional 58 cm) many of the soil particles were arranged bodies composed of unique horizontal layers. into blocklike aggregrates. When moist soil from These naturally occurring bodies are soils. A rec- layer three was pressed between the fingers, more ognition of the kinds of soil layers and their stickiness was observed than in layers one and properties is required in order to use soils effec- two, which meant that layer three had a greater tively for many different purposes.

11 1 2 SOIL AS A NATURAL BODY

First is the formation of a parent material from which the soil evolves and, second, the evolution of soil layers, as shown in Figure 2.1. Approxi- mately 99 percent of the world's soils develop in mineral parent material that was or is derived from the weathering of bedrock, and the rest develop in organic materials derived from plant growth and consisting of muck or peat.

Bedrock Weathering and Formation of Parent Material Bedrock is not considered soil parent material because soil layers do not form in it. Rather, the unconsolidated debris produced from the weathering of bedrock is soil parent material. When bedrock occurs at or near the land surface, the weathering of bedrock and the formation of parent material may occur simultaneously with the evolution of soil layers. This is shown in Figure 2.2, where a single soil horizon, the topsoil layer, overlies the R layer, or bedrock. The topsoil layer is about 12 inches (30 cm) thick and has evolved slowly at a rate controlled by the rate of rock weathering. The formation of a centimeter of soil in hundreds of years is accurate for this example of soil formation. Rates of parent material formation from the direct weathering of bedrock are highly variable. A weakly cemented sandstone in a humid environment might disintegrate at the rate of a centimeter in 10 years and leave 1 centimeter of soil. Con-

FIGURE 2.2 Rock weathering and the formation of the topsoil layer are occurring simultaneously. Scale is in feet.

FIGURE 2.1 This alfalfa taproot grew vertically downward through the upper three layers. At a depth of 23 inches (58 cm), the taproot encountered an impenetrable layer (layer 4) and grew in a lateral direction above the layer.

THE PARENT MATERIAL OF SOILS Soil formation, or the development of soils that are natural bodies, includes two broad processes. SOIL FORMATION 13 versely, quartzite (metamorphosed sandstone) thick alluvial sediments occur in the valley. Very nearby might weather so slowly that any weath- thick glacial deposits occur on the tree-covered ered material might be removed by water or wind lateral moraine that is adjacent to the valley floor erosion. Soluble materials are removed during along the left side. An intermediate thickness of limestone weathering, leaving a residue of insolu- parent material occurs where trees are growing ble materials. Estimates indicate that it takes below the bare mountaintops and above the thick 100,000 years to form a foot of residue from the alluvial and moraine sediments. Most of the weathering of limestone in a humid region. Where world's soils have formed in sediments consisting soils are underlain at shallow depths by bedrock, of material that was produced by the weathering loss of the soil by erosion produces serious con- of bedrock at one place and was transported and sequences for the future management of the land. deposited at another location. In thick sediments or parent materials, the formation of soil layers is not limited by the rate of rock weathering, and Sediment Parent Materials several soil layers may form simultaneously. Weathering and erosion are two companion and opposing processes. Much of the material lost from a soil by erosion is transported downslope SOIL FORMATION and deposited onto existing soils or is added to some sediment at a lower elevation in the land- Soil layers are approximately parallel to the land scape. This may include alluvial sediments along surface and several layers may evolve simulta- streams and rivers or marine sediments along neously over a period of time. The layers in a soil ocean shorelines. Glaciation produced extensive are genetically related; however, the layers differ sediments in the northern part of the northern from each other in their physical, chemical, and hemisphere. biological properties. In soil terminology, the lay- Four constrasting parent material-soil environ- ers are called horizons. Because soils as natural ments are shown in Figure 2.3. Bare rock is ex- bodies are characterized by genetically developed posed on the steep slopes near the mountaintops. horizons, soil formation consists of the evolution Here, any weathered material is lost by erosion of soil horizons. A vertical exposure of a soil con- and no parent material or soil accumulates. Very sisting of the horizons is a soil profile.

FIGURE 2.3 Four distinct soil- forming environments are depicted in this landscape in the Rocky Mountains, United States. On the highest and steepest slopes, rock is exposed because any weathered material is removed by erosion as fast as it forms. Thick alluvial sediments occur on the valley floor and on the forested lateral moraine adjacent to the valley floor along the left side. Glacial deposits of varying thickness overlying rock occur on the forested mountain slopes at intermediate elevations. 1 4 SOIL AS A NATURAL BODY

Soil-Forming Processes animals feeding on the organic debris eventually Horizonation (the formation of soil horizons) re- die and thus contribute to the formation of hu- sults from the differential gains, losses, transfor- mus. Humus has a black or dark-brown color, mations, and translocations that occur over time which greatly affects the color of A horizons. In within various parts of a vertical section of the areas in which there is abundant plant growth, parent material. Examples of the major kinds only a few decades are required for a surface layer of changes that occur to produce horizons are: to acquire a dark color, due to the humification (1) addition of organic matter from plant growth, and accumulation of organic matter, forming an A mainly to the topsoil; (2) transformation repre- horizon. sented by the weathering of rocks and minerals The uppermost horizons shown in Figures 2.1 and the decomposition of organic matter; (3) loss and 2.2 are A horizons. The A horizon in Figure 2.1 of soluble components by water moving down- was converted into a plow layer by frequent plow- ward through soil carrying out soluble salts; and, ing and tillage. Such A horizons are called Ap (4) translocation represented by the movement of horizons, the p indicating plowing or other distur- suspended mineral and organic particles from the bance of the surface layer by cultivation, pastur- topsoil to the subsoil. ing, or similar uses. For practical purposes, the topsoil in agricultural fields and gardens is synon- ymous with Ap horizon. Formation of A and C Horizons At this stage in soil evolution, it is likely that the Many events, such as the deposition of volcanic upper part of the underlying parent material has ash, formation of spoil banks during railroad con- been slightly altered. This slightly altered upper struction, melting of glaciers and formation of part of the parent material is the C horizon. The glacial sediments, or catastrophic flooding and soil at this stage of evolution has two horizons- formation of sediments have been dated quite the A horizon and the underlying C horizon. Such accurately. By studying soils of varying age, soil soils are AC soils; the evolution of an AC soil is scientists have reconstructed the kinds and the illustrated in Figure 2.4. sequence of changes that occurred to produce soils. Glacial sediments produced by continental and Formation of B Horizons alpine glaciation are widespread in the northern The subsoil in an AC soil consists of the C horizon hemisphere, and the approximate dates of the and, perhaps, the upper part of the parent mate- formation of glacial parent materials are known. rial. Under favorable conditions, this subsoil layer After sediments have been produced near a retreating ice front, the temperature may become favorable for the invasion of plants. Their growth FIGURE 2.4 Sequential evolution of some soil results in the addition of organic matter, espe- horizons in a sediment parent material. cially the addition of organic matter at or near the soil surface. Animals, bacteria, and fungi feed on the organic materials produced by the plants, resulting in the loss of much carbon as carbon dioxide. During digestion or decomposition of fresh organic matter, however, a residual organic fraction is produced that is resistant to further alteration and accumulates in the soil. The resistant organic matter is called humus and the process is humification. The microorganisms and

SOIL FORMATION 15

The Bt Horizon Soil parent materials frequently contain calcium carbonate (CaCO 3), or lime, and are alkaline. In the case of glacial parent materials, lime was incorporated into the ice when glaciers overrode limestone rocks. The subsequent melting of the ice left a sediment that contains limestone particles. In humid regions, the lime dissolves in percolating water and is removed from the soil, a process called leaching. Leaching effects are progressive from the surface downward. The surface soil first becomes acid, and subsequently leaching produces an acid subsoil. An acid soil environment greatly stimulates mineral weathering or the dissolution of minerals with the formation of many ions. The reaction of orthoclase feldspar (KAISiO 3 ) with water and H+ FIGURE 2.5 A soil scientist observing soil is as follows: properties near the boundary between the A and B horizons in a soil with A, B, and C horizons. As roots + grow downward, or as water percolates downward, 2 KAISiO3 + 9H 2 O + 2H they encounter a different environment in the A, B, (orthoclase) and C horizons. (Photograph USDA.) = H 4AI 2 Si 2 0 9 + 2K+ + 4H 4 Si04 (kaolinite) (silicic acid)

eventually develops a distinctive color and some The weathering reaction illustrates three impor- other properties that distinguish it from the A hori- tant results of mineral weathering. First, clay parti- zon and underlying parent material, commonly at cles (fine-sized mineral particles) are formed-in a depth of about 60 to 75 centimeters. This altered the example, kaolinite. In effect, soils are "clay subsoil zone becomes a B horizon and develops factories. Second, ions are released into the soil as a layer sandwiched between the A and a new solution, including nutrient ions such as K + . deeper C horizon. At this point in soil evolution, Third, other compounds (silicic acid) of varying i nsufficient time has elapsed for the B horizon to solubility are formed and are subject to leaching have been significantly enriched with fine-sized and removal from the soil. (colloidal) particles, which have been translo- Clay formation results mainly from chemical cated downward from the A horizon by percolat- weathering. Time estimates for the formation of 1 ing water. Such a weakly developed B horizon is percent clay inn rock parent material range from given the symbol w (as in Bw), to indicate its 500 to 10,000 years. Some weathered rocks with weakly developed character. A Bw horizon can be small areas in which minerals are being con- distinguished from A and C horizons primarily by verted into clay are shown in Figure 2.6. color, arrangement of soil particles, and an inter- Many soil parent materials commonly contain mediate content of organic matter. A soil with A, some clay. Some of this clay, together with clay B, and C horizons is shown in Figure 2.5. produced by weathering during soil formation, During the early phases of soil evolution, the tends to be slowly translocated downward from soil formation processes progressively transform the A horizon to the B horizon by percolating parent material into soil, and the soil increases in water. When a significant increase in the clay thickness. The evolution of a thin AC soil into a content of a Bw horizon occurs due to clay trans- thick ABwC soil is illustrated in Figure 2.4. location, a Bw horizon becomes a Bt horizon. 1 6 SOIL AS A NATURAL BODY

cles are believed to disperse when dry soil is wetted at the end of a dry season and the clay particles migrate downward in percolating water during the wet season. When the downward percolating water encounters dry soil, water is with- drawn into the surrounding dry soil, resulting in the deposition of clay on the walls of pore spaces. Repeated cycles of wetting and drying build up layers of oriented clay particles, which are called clay skins. Many studies of clay illuviation have been made. The studies provide evidence that thousands of years are needed to produce a significant FIGURE 2.6 Weathering releases mineral grains in increase in the content of clay in B horizons. An rocks and results in the formation of very fine-sized particles of clay, in this case, kaolinite. example is the study of soils on the alluvial floodplain and adjacent alluvial fans in the Cen- tral Valley of California. Here, increasing eleva- Thin layers or films of clay can usually be ob- tion of land surfaces is associated with increasing served along cracks and in pore spaces with a age. The soils studied varied in age from 1,000 to 10-power hand lens. The process of accumulation more than 100,000 years. of soil material into a horizon by movement out of The results of the study are presented in Figure some other horizon is illuviation. The t (as in Bt) 2.7. The Hanford soil developed on the floodplain refers to an illuvial accumulation of clay. The Bt is 1,000 years old; it shows no obvious evidence of horizon may be encountered when digging holes illuviation of clay. The 10,000-year-old Greenfield for posts or trenching for laying underground soil has about 1.4 times more clay in the subsoil pipes. (Bt horizon) than in the A horizon. Snelling soils Alternating periods of wetting and drying seem are 100,000 years old and contain 2.5 times more necessary for clay translocation. Some clay parti- clay in the Bt horizon than in the A horizon. The

FIGURE 2.7 Clay distribution as a function of time in soils developed from granitic parent materials in the Central Valley of California. The Hanford soil, only 1,000 years old, does not have a Bt horizon. The other three soils have Bt horizons. The Bt horizon of the San Joaquin is a claypan that inhibits roofs and the downward percolation of water. (After Arkley, 1964.) SOIL FORMATION 17

San Joaquin soil is 140,000 years old, and has 3.4 The high content of sand results in soils with times more clay in the horizon of maximum clay low fertility and low water-retention capacity accumulation as compared to the A horizon. (droughtiness). The three youngest soils (Hanford, Greenfield, and Snelling) are best suited for agriculture because the subsoil horizons are permeable to wa- Formation of E Horizons ter and air, and plant roots penetrate through the B The downward translocation of colloids from the horizons and into the C horizons. Conversely, the A horizon may result in the concentration of sand impermeable subsoil horizon in San Joaquin soil and silt-sized particles (particles larger than clay causes shallow rooting. The root zone above the size) of quartz and other resistant minerals in the impermeable horizon becomes water saturated in upper part of many soils. In soils with thin A the wet seasons. The soil is dry and droughty in horizons, a light-colored horizon may develop at the dry season. the boundary of the A and B horizons (see Figure Water aquifiers underlie soils and varying thick- 2.4). This horizon, commonly grayish in color, is nesses of parent materials and rocks. Part of the the E horizon. The symbol E is derived from eluvi- precipitation in humid regions migrates com- ation, meaning, "washed-out." Both the A and E pletely through the soil and recharges underlying horizons are eluvial in a given soil. The main aquifers. The development of water-impermeable feature of the A horizon, however, is the presence claypans over an extensive region results in less of organic matter and a dark color, whereas that of water recharge and greater water runoff. This has the E horizon is a light-gray color and having low occurred near Stuttgart, Arkansas, where wells organic matter content and a concentration of silt used for the irrigation of rice have run dry because and sand-sized particles of quartz and other resis- of the limited recharge of the aquifer. tant minerals. The development of E horizons occurs more readily in forest soils than in grassland soils, be- The Bhs Horizon Many sand parent materials cause there is usually more eluviation in forest contain very little clay, and almost no clay forms soils, and the A horizon is typically much thinner. in them via weathering. As a consequence, clay The development of E horizons occurs readily in illuviation is insignificant and Bt horizons do not soils with Bhs horizons, and the E horizons may evolve. Humus, however, reacts with oxides of have a white color (see soil on book cover). aluminum and/or iron to form complexes in the A soil with A, E, Bt, and C horizons is shown in upper part of the soil. Where much water for Figure 2.8. At this building site, the suitability of leaching (percolation) is present, as in humid the soil for the successful operation of a septic regions, these complexes are translocated down- effluent disposal system depends on the rate at ward in percolating water to form illuvial accumu- which water can move through the least per- lations in the B horizon. The illuvial accumulation meable horizon, in this example the Bt hori- of humus and oxides of aluminum and/or iron in zon. Thus, the value of rural land for home the B horizon produces Bhs horizons. The h indi- construction beyond the limits of municipal cates the presence of an illuvial accumulation of sewage systems depends on the nature of the humus and the s indicates the presence of illuvial subsoil horizons and their ability to allow for oxides of aluminum and/or iron. The symbol s is the downward migration and disposal of sewage derived from sesquioxides (such as Fe 2 O 3 and effluent. Suitable sites for construction can A1 2 O3). Bhs horizons are common in very sandy be identified by making a percolation test of soils that are found in the forested areas of the those horizons through which effluent will be eastern United States from Maine to Florida. disposed. 1 8 SOIL AS A NATURAL BODY

FIGURE 2.8 A soil with A, E, Bt, and C horizons that formed in 10,000 years under forest vegetation. The amount of clay in the Bt horizon and permeability of the Bt horizon to water determine the suitability of this site for home construction in a rural area where the septic tank effluent must be disposed by percolating through the soil.

Formation of 0 Horizons The Soil-Forming Factors Vegetation produced in the shallow waters of Five soil-forming factors are generally recognized: lakes and ponds may accumulate as sediments of parent material, organisms, climate, topography, peat and muck because of a lack of oxygen in the and time. It has been shown that Bt and Bhs water for their decomposition. These sediments horizon development is related to the clay and are the parent material for organic soils. Organic sand content within the parent material and/or soils have 0 horizons; the O refers to soil layers the amount of clay that is formed during soil evo- dominated by organic material. In some cases, lution. extreme wetness and acidity at the surface of the Grass vegetation contributes to soils with thick soil produce conditions unfavorable for decom- A horizons because of the profuse growth of fine position of organic matter. The result is the forma- roots in the upper 30 to 40 centimeters of soil. In tion of O horizons on the top of mineral soil hori- forests, organic matter is added to soils mainly by zons. Although a very small proportion of the leaves and wood that fall onto the soil surface. world's soils have O horizons, these soils are Small-animal activities contribute to some mixing widely scattered throughout the world. of organic matter into and within the soil. As a result, organic matter in forest soils tends to be incorporated into only a thin layer of soil, resulting in thin A horizons. SOILS AS NATURAL BODIES The climate contributes to soil formation Various factors contribute to making soils what through its temperature and precipitation com- they are. One of the most obvious is parent mate- ponents. If parent materials are permanently fro- rial. Soil formation, however, may result in many zen or dry, soils do not develop. Water is needed different kinds of soils from a given parent mate- for plant growth, for weathering, leaching, and rial. Parent material and the other factors that are translocation of clay, and so on. A warm, humid responsible for the development of soil are the climate promotes soil formation, whereas dry oil-foming faco. and/or cold climates inhibit it. SOILS AS NATURAL BODIES 1 9

The topography refers to the general nature of the land surface. On slopes, the loss of water by runoff and the removal of soil by erosion retard soil formation. Areas that receive runoff water may have greater plant growth and organic matter content, and more water may percolate through the soil. The extent to which these factors operate is a function of the amount of time that has been available for their operation. Thus, soil may be defined as:

nconolidaed maeial on he face of he eah ha ha been bjeced o and inflenced FIGURE 2.10 Soil scientists studying soils as b he geneic and enionmenal faco of pa- natural bodies in the field. Soils are exposed in the en maeial, climae, oganim, and opogaph, pit and soils data are displayed on charts for all acing oe a peiod of ime. observation and discussion.

slope (topography) account for the existence of Si Bdie a Pa f Ladcae different soil bodies in a given field, as shown in At any given location on the landscape, there is a Figure 2.9. The dark-colored soil in the fore- particular soil with a unique set of properties, ground receives runoff water from the adjacent including kinds and nature of the horizons. Soil slopes. The light-colored soil on the slopes devel- properties may remain fairly constant from that oped where water runoff and erosion occurred. location in all directions for some distance. The Distinctly different management practices are re- area in which soil properties remain reasonably quired to use effectively the poorly drained soil in constant is a soil body. Eventually, a significant the foreground and the eroded soil on the slope. change will occur in one or more of the soil- The boundary between the two different soils forming factors and a different soil or soil body soils is easily seen. In many instances the bound- will be encountered. aries between soils require an inspection of the Locally, changes in parent material and/or soil, which is done by digging a pit or using a soil auger. FIGURE 2.9 A field or landscape containing black- colored soil in the lowest part of the landscape. This soil receives runoff water and eroded H Sciei Sd Si a sediment from the light-colored soil on the sloping areas. Naa Bdie A particular soil, or soil body, occupies a particular part of a landscape. To learn about such a soil, a pit is usually dug and the soil horizons are described and sampled. Each horizon is described in terms of its thickness, color, arrangement of particles, clay content, abundance of roots, presence or absence of lime, pH, and so on. Samples from each horizon are taken to the laboratory and are analyzed for their chemical, physi- 20 SOIL AS A NATURAL BODY

FIGURE 2.11 Muck (organic) soil layer being replaced with sand to increase the stability of the roadbed. cal, and biological properties. These data are pre- moved and replaced with material that can with- sented in graphic form to show how various soil stand the pressures of vehicular traffic. The scene properties remain the same or change from one in Figure 2.11 is of section of road that was built horizon to another (shown for clay content in without removing a muck soil layer. The road Figure 2.7). Figure 2.10 shows soil scientists became unstable and the muck layer was eventu- studying a soil in the field. Pertinent data are ally removed and replaced with sand. presented by a researcher, using charts, and the properties and genesis of the soil are discussed by the group participitants. SUMMARY The original source of all mineral soil parent ma- Importance of Concept of Soils as terial is rock weathering. Some soils have formed Natural Bodies directly in the products of rock weathering at their The nature and properties of the horizons in a soil present location. In these instances, horizon for- determine the soil's suitability for various uses. To mation may be limited by the rate of rock weather- use soils prudently, an inventory of the soil's ing, and soil formation may be very slow. Most properties is needed to serve as the basis for mak- soils, however, have formed in sediments resulting predictions of soil behavior in various situa- ing from the erosion, movement, and deposition tions. Soil maps, which show the location of the of material by glaciers, water, wind, and gravity. soil bodies in an area, and written reports about Soils that have formed in organic sediments are soil properties and predictions of soil behavior for organic soils. various uses began in the United States in 1896. By The major soil-forming processes include: the 1920s, soil maps were being used to plan the (1) humification and accumulation of organic location and construction of highways in Michi- matter, (2) rock and mineral weathering, (3) gan. Soil materials that are unstable must be re- leaching of soluble materials, and (4) the eluvi- REFERENCES 2 1 ation and illuviation of colloidal particles. The REFERENCES operation of the soil formation processes over Arkley, R. J. 1964. Soil Survey of the Eastern Stanislaus time produces soil horizons as a result of differen- Aea, Califonia. U.S.D.A. and Cal. Agr. Exp. Sta. tial changes in one soil layer, as compared to Barshad, I. 1959. "Factors Affecting Clay Formation." another. Clays and Clay Minerals. E. Ingerson (ed.), Earth The master soil horizons or layers include the Science Monogaph 2. Pergamon, New York. O, A, E, B, C, and R horizons. Jenny, H. 1941. Faco of Soil Fomaion. McGraw-Hill, Different kinds of soil occur as a result of the New York. interaction of the soil-forming factors: parent ma- Jenny, H. 1980. The Soil Reoce. Springer-Verlag, terial, organisms, climate, topography, and time. New York. Landscapes are composed of three-dimen- Simonson, R. W. 1957. "What Soils Are." USDA Agricul- sional bodies that have naturally (genetically) de- tural Yearbook, Washington, D. C. Simmonson, R. W. 1959. "Outline of Generalized The- veloped horizons. These bodies are called soils. ory of Soil Genesis." Soil Sci. Soc. Am. Poc. 23:161- Prudent use of soils depends on a recognition 164. of soil properties and predictions of soil behavior Twenhofel, W, H. 1939. "The Cost of Soil in Rock and under various conditions. Time." Am J. Sci. 237:771-780. CHAPTER 3

SOIL PHYSICAL PROPERTIES

Physically, soils are composed of mineral and the size groups that are smaller than gravel. The organic particles of varying size. The particles are diameter and the number and surface area per arranged in a matrix that results in about 50 per- gram of the separates are given in Table 3.1. cent pore space, which is occupied by water and Sand is the 2.0 to 0.05 millimeter fraction and, air. This produces a three-phase system of solids, according to the United States Department of Agri- liquids, and gases. Essentially, all uses of soils are culture (USDA) system, the sand fraction is sub- greatly affected by certain physical properties. divided into very fine, fine, medium, coarse, and The physical properties considered in this chapter very coarse sand separates. Silt is the 0.05 to 0.002 include: texture, structure, consistence, porosity, millimeter (2 microns) fraction. At the 0.05 milli- density, color, and temperature. meter particle size separation, between sand and silt, it is difficult to distinguish by feel the individ- SOIL TEXTURE ual particles. In general, if particles feel coarse or abrasive when rubbed between the fingers, the The physical and chemical weathering of rocks particles are larger than silt size. Silt particles feel and minerals results in a wide range in size of smooth like powder. Neither sand nor silt is sticky particles from stones, to gravel, to sand, to silt, when wet. Sand and silt differ from each other on and to very small clay particles. The particle-size the basis of size and may be composed of the distribution determines the soil's coarseness or same minerals. For example, if sand particles are fineness, or the soil's texture. Specifically, texture smashed with a hammer and particles less than is the relative proportions of sand, silt, and clay in 0.05 millimeters are formed, the sand has been a soil. converted into silt. The sand and silt separates of many soils are The Soil Separates dominated by quartz. There is usually a signifi- Soil separates are the size groups of mineral parti- cant amount of weatherable minerals, such as cles less than 2 millimeters (mm) in diameter or feldspar and mica, that weather slowly and re-

22 SOIL TEXTURE 23

TABLE 3.1 Some Characteristics of Soil Separates Surface Number of Area in Diameter, Diameter, Particles I Gram, 2 Separate mm' mmb per Gram cm Very coarse sand 2.00-1.00 90 11 Coarse sand 1.00-0.50 2.00-0.20 720 23 Medium sand 0.50-0.25 5,700 45 Fine sand 0.25-0.10 0.20-0.02 46,000 91 Very fine sand 0.10-0.05 722,000 227 Silt 0.05-0.002 0.02-0.002 5,776,000 454 Clay Below 0.002 Below 0.002 90,260,853,000 8,000,000`

a United States Department of Agriculture System. b International Soil Science Society System. The surface area of platy-shaped montmorillonite clay particles determined by the glycol retention method by Sor and Kemper. (See Soil Science Society of America Proceedings, Vol. 23, p. 106, 1959.) The number of particles per gram and surface area of silt and the other separates are based on the assumption that particles are spheres and the largest particle size permissible for the separate.

lease ions that supply plant needs and recombine statements apply to most clay in soils, some soil to form secondary minerals, such as clay. The clays have little tendency to show stickiness and greater specific surface (surface area per gram) of expand when wetted. The clay fraction usually silt results in more rapid weathering of silt, com- has a net negative charge. The negative charge 2+' 2 +, pared with sand, and greater release of nutrient adsorbs nutrient cations, including Ca Mg ions. The result is a generally greater fertility in and K + , and retains them in available form for use soil having a high silt content than in soils high in by roots and microbes. sand content. Clay particles have an effective diameter less than 0.002 millimeters (less than 2 microns). They tend to be plate-shaped, rather than sperical, and FIGURE 3.1 An electron micrograph of clay very small in size with a large surface area per particles magnified 35,000 times. The platy shape, or gram, as shown in Table 3.1. Because the specific flatness, of the particles results in very high specific surface of clay, is many times greater than that of surface. (Photograph courtesy Mineral Industries Experiment Station, Pennsylvania State University.) sand or silt, a gram of clay adsorbs much more water than a gram of silt or sand, because water adsorption is a function of surface area. The clay particles shown in Figure 3.1 are magnified about 30,000 times; their plate-shaped nature contributes to very large specific surface. Films of water between plate-shaped clay particles act as a lubri- cant to give clay its plasticity when wet. Con- versely, when soils high in clay are dried, there is an enormous area of contact between plate- shaped soil particles and great tendency for very hard soil clods to form. Although the preceding 24 SOIL PHYSICAL PROPERTIES

Particle Size Analysis is poured into a specially designed cylinder, and Sieves can be used to separate and determine the distilled water is added to bring the contents up to content of the relatively large particles of the sand volume. and silt separates. Sieves, however, are unsatis- The soil particles settle in the water at a speed factory for the separation of the clay particles from directly related to the square of their diameter and the silt and sand. The hydrometer method is an inversely related to the viscosity of the water. A empirical method that was devised for rapidly hand stirrer is used to suspend the soil particles determining the content of sand, silt, and clay in a thoroughly and the time is immediately noted. A soil. specially designed hydrometer is carefully in- In the hydrometer method a sample (usually 50 serted into the suspension and two hydrometer grams) of air-dry soil is mixed with a dispersing readings are made. The sand settles in about 40 agent (such as a sodium pyrophosphate solution) seconds and a hydrometer reading taken at 40 for about 12 hours to promote dispersion. Then, seconds determines the grams of silt and clay the soil-water suspension is placed in a metal cup remaining in suspension. Subtraction of the 40- with baffles on the inside, and stirred on a mixer second reading from the sample weight gives the for several minutes to bring about separation of grams of sand. After about 8 hours, most of the silt the sand, silt, and clay particles. The suspension has settled, and a hydrometer reading taken at 8 hours determines the grams of clay in the sample (see Figure 3.2). The silt is calculated by differ- FIGURE 3.2 Inserting hydrometer for the 8-hour ence: add the percentage of sand to the percent- reading. The sand and silt have settled. The 8-hour age of clay and subtract from 100 percent. reading measures grams of clay in suspension and is used to calculate the percentage of clay. PROBLEM: Calculate the percentage of sand, clay, and silt when the 40-second and 8-hour hydrometer readings are 30 and 12, respectively; assume a 50 gram soil sample is used:

sample weight - 40-second reading x 100 = %sand sample weight 50 g - 30 g x 100 = 40% sand 509 8-hour reading x 100 = % clay sample weight 1 2 g x 100 = 24% clay 50 g 1 00% - (40% + 24%) = 36% silt

After the hydrometer readings have been obtained, the soil-water mixture can be poured over a screen to recover the entire sand fraction. After it is dried, the sand can be sieved to obtain the various sand separates listed in Table 3.1. SOIL TEXTURE 25

FIGURE 3.3 The textural triangle shows the limits of sand, silt, and clay contents of the various texture classes. Point A represents a soil that contains 65 percent sand, 15 percent clay, and 20 percent silt, and is a sandy loam.

Soil Tetural Classes arates-sand, silt, and clay. For sandy soils (sand Once the percentages of sand, silt, and clay have and loamy sand), the properties and use of the been determined, the soil can be placed in one of soil are influenced mainly by the sand content of 12 major textural classes. For a soil that contains the soil. For clays (sandy clay, clay, silty clay), the 15 percent clay, 65 percent sand, and 20 percent properties and use of the soil are influenced silt, the logical question is: What is the texture or mainly by the high clay content. textural class of the soil? The texture of a soil is expressed with the use of class names, as shown in Figure 3.3. The sum of Determining Teture b the the percentages of sand, silt, and clay at any point Field Method in the triangle is 100. Point A represents a soil When soil scientists make a soil map, they use the containing 15 percent clay, 65 percent sand, and field method to determine the texture of the soil 20 percent silt, resulting in a textural class name horizons to distinguish between different soils. of sandy loam. A soil containing equal amounts of When investigating a land-use problem, the ability sand, silt, and clay is a clay loam. The area out- to estimate soil texture on location is useful in lined by the bold lines in the triangle defines a diagnosing the problem and in formulating a so- given class. For example, a loam soil contains 7 to lution. 27 percent clay, 28 to 50 percent silt, and between A small quantity of soil is moistened with water 22 and 52 percent sand. Soils in the loam class and kneaded to the consistency of putty to deter- are influenced almost equally by all three sep- mine how well the soil forms casts or ribbons 26 SOIL PHYSICAL PROPERTIES

(plasticity). The kind of cast or ribbon formed is related to the clay content and is used to catego- rize soils as loams, clay loams, and clays. This is shown in Figure 3.4. If a soil is a loam, and feels very gritty or sandy, it is a sandy loam. Smooth-feeling loams are high in silt content and are called silt loams. If the sample is intermediate, it is called a loam. The same applies to the clay loams and clays. Sands are loose and incoherent and do not form ribbons. More detailed specifications are given in Appendix 1.

I nfluence of Coarse Fragments on Class Names Some soils contain significant amounts of gravel and stones and other coarse fragments that are larger than the size of sand grains. An appropriate adjective is added to the class name in these cases. For example, a sandy loam in which 20 to 50 percent of the volume is made up of gravel is a gravelly sandy loam. Cobbly and stony are used for fragments 7.5 to 25 centimeters and more than 25 centimeters in diameter, respectively. Rocki- FIGURE 3.5 Foundation soil with high clay content ness expresses the amount of the land surface expanded and shrunk with wetting and drying, that is bedrock. cracking the wall. (Photograph courtesy USDA.)

Teture and the Use of Soils closely related to soil texture. Many clay soils water permeability, ease of tillage, Plasticity, expand and shrink with wetting and drying, caus- droughtiness, fertility, and productivity are all ing cracks in walls and foundations of buildings, as shown in Figure 3.5. Conversely, many red- FIGURE 3.4 Determination of texture by the field colored tropical soils have clay particles com- method. Loam soil on left forms a good cast when posed mainly of kaolinite and oxides of iron and moist. Clay loam in center forms a ribbon that breaks somewhat easily. Clay on the right forms a long alumimum. These particles have little capacity to fexible ribbon. develop stickiness and to expand and contract on wetting and drying. Such soils can have a high clay content and show little evidence of stickiness when wet, or expansion and contraction with wetting and drying. Many useful plant growth and soil texture relationships have been established for certain geographic areas. Maximum productivity for red pine (Pinus resinosa) occurs on coarser-textured soil than for corn (Zea maize) in Michigan. The greatest growth of red pine occurs on soils with sandy SOIL STRUCTURE 27 loam texture where the integrated effects of the roots in subsoil Bt horizons. Water saturation of nutrients, water, and aeration are the most desir- the soil above the Bt horizon may occur in wet able. Loam soils have the greatest productivity for seasons. Shallow rooting may result in a defi- corn. When soils are irrigated and highly fertil- ciency of water in dry seasons. A Bt horizon with a ized, however, the highest yields of corn occur on high clay content and very low water permeability the sand soils. The world record corn yield, set in is called a claypan. A claypan is a dense, compact 1977, occurred on a sandy soil that was fertilized layer in the subsoil, having a much higher clay and irrigated. content than the overlying material from which it The recommendations for reforestation in Ta- is separated by a sharply defined boundary (see ble 3.2 assume a uniform soil texture exists with Figure 3.6). increasing soil depth, but this is often not true in Claypan soils are well suited for flooded rice the field. Species that are more demanding of production. The claypan makes it possible to water and nutrients can be planted if the underly- maintain flooded soil conditions during the growing soil horizons have a finer texture. In some ing season with a minimum amount of water. instances an underlying soil layer inhibits root Much of the rice produced in the United States is penetration, thus creating a shallow soil with lim- produced on claypan soils located on nearly level ited water and nutrient supplies. land in Arkansas, Louisiana, and Texas. The other Young soils tend to have a similar texture in major kind of rice soil in the United States has very each horizon. As soils evolve and clay is translo- limited water permeability because there is a clay cated to the B horizon, a decrease in permeability texture in all horizons. of the subsoil occurs. Up to a certain point, however, an increase in the amount of clay in the subsoil is desirable, because the amount of water and nutrients stored in that zone can also be in- SOIL STRUCTURE creased. By slightly reducing the rate of water Texture is used in reference to the size of soil movement through the soil, fewer nutrients will particles, whereas structure is used in reference to be lost by leaching. The accumulation of clay in the arrangement of the soil particles. Sand, silt, time may become sufficient to restrict severely the and clay particles are typically arranged into sec- movement of air and water and the penetration of ondary particles called peds, or aggregates. The

TABLE 3.2 Recommendations for Reforestation on Upland Soils in Wisconsin in Relation to Soil Textureª Percent Silt Plus Clay Species Recommended Less than 5 (Only for wind erosion control) 5-10 Jack pine, Red cedar 10-15 Red pine, Scotch pine, Jack pine 15-25 Increasing All pines 25-35 demand for White pine, European larch, Yellow birch, White water and elm, Red oak, Shagbark hickory, Black locust Over 35 nutrients White spruce, Norway spruce, White cedar, White ash, Basswood, Hard maple, White oak, Black walnut

ª Wilde, S. A., "The Significance of Soil Texture in Forestry, and Its Determination by a Rapid Field Method," Journal of Forestry, 33:503-508, 1935. By permission Journal of Forestry. 28 SOIL PHYSICAL PROPERTIES shape and size of the peds determine the soil's and the downward movement of water, however, structure. are not inhibited in the B horizon of most soils.

Importance of Structure Genesis and Tpes of Structure Structure modifies the influence of texture with Soil peds are classified on the basis of shape. The regard to water and air relationships and the ease four basic structural types are spheroid, platelike, of root penetration. The macroscopic size of most blocklike, and prismlike. These shapes give rise peds results in the existence of interped spaces to granular, platy, blocky, and prismatic types of that are much larger than the spaces existing be- structure. Columar structure is prismatic-shaped tween adjacent sand, silt, and clay particles. Note peds with rounded caps. the relatively large cracks between the structural Soil structure generally develops from material peds in the claypan (Bt horizon) shown in Figure that is without structure. There are two structure- 3.6. The peds are large and in the shape of blocks less conditions, the first of which is sands that or prisms, as a result of drying and shrinkage remain loose and incoherent. They are referred to upon exposure. When cracks are open, they are as single grained. Second, materials with a signif- avenues for water movement. When this soil wets, icant clay content tend to be massive if they do however, the clay expands and the cracks be- not have a developed structure. Massive soil has tween the peds close, causing the claypan hori- no observable aggregation or no definite and or- zon to become impermeable. Root penetration derly arrangement of natural lines of weakness.

FIGURE 3.6 A soil with a strongly developed Bt horizon; a claypan. On drying, cracks form between structural units. On wetting, the soil expands, the cracks close, and the claypan becomes impermeable to water and roots. (Photograph courtesy Soil Conservation Service, USDA.) SOIL STRUCTURE 29

Massive soil breaks up into random shaped clods 0. Structureless-no observable aggregration or or chunks. Structure formation is illustrated in no definite and orderly arrangement of natu- Figure 3.7. ral lines of weakness. Massive, if coherent, as Note in Figure 3.8 that each horizon of the soil in clays. Single grained, if noncoherent, as in has a different structure. This is caused by differ- sands. ent conditions for structure formation in each ho- 1. Weak-poorly formed indistinctive peds, rizon. The A horizon has the most abundant root barely observable in place. and small-animal activity and is subject to fre- 2. Moderate-well-formed distinctive peds, quent cycles of wetting and drying. The structure moderately durable and evident, but not dis- tends to be granular (see Figure 3.8). The Bt hori- tinct in undisturbed soil. zon has more clay, less biotic activity, and is 3. Strong-durable peds that are quite evident under constant pressure because of the weight of in undisturbed soil, adhere weakly to one an- the overlying soil. This horizon is more likely to other, and become separated when the soil is crack markedly on drying and to develop either a disturbed. blocky or prismatic structure. The development of The sequence followed in combining the three structure in the E horizon is frequently weak and terms to form compound names is first the grade, the structure hard to observe. In some soils the E then the class, and finally the type. An example of horizon has a weakly developed platy structure a soil structural description is "strong fine (see Figure 3.8). granular."

Grade and Class Managing Soil Structure Complete descriptions of soil structure include: Practical management of soil structure is gener- (1) the type, which notes the shape and arrange- ally restricted to the topsoil or plow layer in regard ment of peds; (2) the class, which indicates ped to use of soils for plant growth. A stable structure size; and (3) the grade, which indicates the dis- at the soil surface promotes more rapid infil- tinctiveness of the peds. The class categories de- tration of water during storms and results in re- pend on the type. A table of the types and classes duced water runoff and soil erosion and greater of soil structure is given in Appendix 2. Terms for water storage within the soil. The permanence of grade of structure are as follows: peds depends on their ability to retain their shape

FIGURE 3.7 Illustration of the formation of various types of soil structure from single- grained and massive soil conditions. Structure types are based on the shape of aggregates. 30 SOIL PHYSICAL PROPERTIES

-- 30-

- 60 --

-- 9O--

-- l2O--

FIGURE 3.8 Structure of the horizons of a forest soil with a Bt horizon. Scale is in centimeters. SOIL CONSISTENCE 3 1 after being subjected to the disruptive effects of stability. Practices that result in keeping a vegeta- raindrops and tillage. In addition, for peds to be tive cover also reduce raindrop impact and thus permanent, the particles in the peds must hold reduce the disruption of peds. together when dry peds are wetted. When dry soil is suddenly wetted, water moves into the peds from all directions and compresses SOIL CONSISTENCE the air in the pore spaces. Peds unable to with- Consistence is the resistance of the soil to defor- stand the pressure exerted by the entrapped air mation or rupture. It is determined by the cohe- are disrupted and fall apart. Unstable peds at the sive and adhesive properties of the entire soil soil surface in cultivated fields are also broken mass. Whereas structure deals with the shape, apart by the force of falling raindrops. Disruption size, and distinctiveness of natural soil aggre- of peds produces smaller chunks and some indi- gates, consistence deals with the strength and vidual particles, which float over the soil surface nature of the forces between the sand, silt, and and seal off the surface to additional infiltrating clay particles. Consistence is important for tillage water. On drying, the immediate surface soil may and traffic considerations. Dune sand exhibits acquire a massive or structureless condition. minimal cohesive and adhesive properties, and Researchers at the University of Wisconsin because sand is easily deformed, vehicles can measured the development of peds in four soils in easily get stuck in it. Clay soils can become sticky which they also determined the content of agents when wet, and thus make hoeing or plowing dif- known to bind particles together and give peds ficult. stability. These binding agents were microbial gum, organic carbon, iron oxide, and clay. The data in Table 3.3 show that microbial gum was Soil Consistence Terms the most important in promoting ped formation. Consistence is described for three moisture lev- Iron oxide, formed during weathering from min- els: wet, moist, and dry. A given soil may be sticky erals that contain iron, was most important in one when wet, firm when moist, and hard when dry. A soil and second most important when the four partial list of terms used to describe consistence includes: soils were grouped together. Soil management practices that result in frequent additions of or- 1. Wet soil-nonsticky, sticky, nonplastic, ganic matter to the soil in the form of plant resi- plastic due, or manure, will tend to produce more micro- 2. Moist soil-loose, friable, firm bial gum and increase ped formation and 3. Dry soil-loose, soft, hard

TABLE 3.3 Order of Importance of Soil Constituents in Formation of Peds over 0.5 mm in Diameter in the A Horizons of Four Wisconsin Soils Soil Order of Importance of Soil Constituents Parr silt loam Microbial gum > clay > iron oxide > organic carbon Almena silt loam Microbial gum > iron oxide Miami silt loam Microbial gum > iron oxide > organic carbon Kewaunee silt loam Iron oxide > clay > microbial gum All soils Microbial gum > iron oxide > organic carbon > clay

Adapted from Chesters, G., O. J., Attoe, and O. N., Allen, "Soil Aggregation in Relation to Various Soil Constituents," Soil Science Societ of America Proceedings Vol. 21, 1957, p. 276, by permission of the Soil Science Society of America. 32 SOIL PHYSICAL PROPERTIES

Plastic soil is capable of being molded or de- Bulk samples of soil horizons are routinely col- formed continuously and permanently, by rela- lected in the field and are used for chemical anal- tively moderate pressure, into various shapes yses and for some physical analyses, such as when wet. Friable soils readily break apart and are particle-size distribution. Soil cores are collected not sticky when moist. to determine soil bulk density. The soil core Two additional consistence terms for special samples are collected to obtain a sample of soil situations are cemented and indurated. Cementa- that has the undisturbed density and pore space tion is caused by cementing agents such as cal- representative of field conditions. Coring ma- cium carbonate, silica, and oxides of iron and chines are used to remove soil cores with a diam- aluminum. Cemented horizons are not affected by eter of about 10 centimeters to a depth of a meter water content and limit root penetration. When a or more. The technique for hand sampling of indi- cemented horizon is so hard that a sharp blow of a vidual soil horizons is shown in Figure 3.9. Care hammer is required to break the soil apart, the soil is considered to be indurated. A silica cemented horizon is called a duripan. FIGURE 3.9 Technique for obtaining soil cores that Large rippers pulled by tractors are used to break are used to determine bulk density. The light-colored up duripans to increase rooting depth in soils. metal core fits into the core sampler just to the left. Sometimes a layer with an accumulation of car- This unit is then driven into the soil in pile-driver bonates (a Ck horizon, for example) accumulates fashion, using the handle and weight unit. so much calcium carbonate as to become cemented or indurated, and is transformed into a petrocalcic horizon. Petrocalcic horizons are also called caliche. Cemented and indurated horizons tend to occur in soils of great age.

DENSITY AND WEIGHT RELATIONSHIPS

Two terms are used to express soil density. Parti- cle density is the average density of the soil particles, and bulk density is the density of the bulk soil in its natural state, including both the particles and pore space.

Particle Density and Bulk Density A soil is composed of mineral and organic particles of varying composition and density. Feldspar minerals (including orthoclase) are the most common minerals in rocks of the earth's crust and very common in soils. They have densities ranging from 2.56 g/cm 3 to 2.76 g/cm3 . Quartz is also a common soil mineral; it has a density of 2.65 g/cm3 . Most mineral soils consist mainly of a wide variety of minerals and a small amount of organic matter. The average particle density for mineral soils is usually given as 2.65 g/cm 3 . DENSITY AND WEIGHT RELATIONSHIPS 33 must be exercised in the collection of soil cores so that the natural structure is preserved. Any change in structure, or compaction of soil, will alter the amount of pore space and, therefore, will alter the bulk density. The bulk density is the mass per unit volume of oven dry soil, calculated as follows: mass oven dry bulk density = volume

PROBLEM: If the soil in the core of Figure 3.9 weighs 600 grams over dry and the core has a volume of 400 cm3 , calculate the bulk density.

bulk density = 400 = 1.5 g/cm3 0cm3 The bulk density of a soil is inversely related to porosity. Soils without structure, such as single- grained and massive soil, have a bulk density of about 1.6 to 1.7 g/cm 3 . Development of structure FIGURE 3.10 Bulk density and pore space of the results in the formation of pore spaces between horizons of a soil with A, E, Bt, and C horizons. peds (interped spaces), resulting in an increase in (Data for Miami loam from Wascher, 1960.) the porosity and a decrease in the bulk density. A value of 1.3 g/cm3 is considered typical of loam surface soils that have granular structure. As the tion exists, depending on the nature of the or- clay content of surface soils increases, there ganic matter and the moisture content at the time tends to be an increase in structural development of sampling to determine bulk density. Bulk den- and a decrease in the bulk density. As clay accu- sities for organic soils commonly range from 0.1 mulates in B horizons, however, the clay fills ex- to 0.6 g/cm 3 . isting pore space, resulting in a decrease in pore Changes in soil porosity due to compaction are space volume. As a result, the formation of Bt commonly evaluated in terms of changes in bulk horizons is associated with an increase in bulk density. density. The bulk densities of the various horizons of a soil are given in Figure 3.10. In this soil the Weight of a Furro-Slice of Soil C horizon has the greatest bulk density,- Soil erosion losses are commonly expressed as 1.7 g/cm3 . The C horizon is massive, or structurel- tons of soil per acre. An average annual soil loss ess, and this is associated with high bulk density of 10 tons per acre is quite meaningless unless the and low porosity. Note that the Bt horizon is more weight of an inch of soil over 1 acre is known. In dense than the A horizon and that there is an this instance, an erosion rate of 10 tons annually inverse relationship between bulk density and to- would result in the loss of a layer about 7 inches tal porosity (macropore space plus micropore thick, every 100 years. space). The acre-furrow-slice has customarily been Organic soils have very low bulk density considered to be a layer of soil about 7 inches 2 compared with mineral soils. Considerable varia- thick over 1 acre. An acre has 43,560 ft . The 34 SOIL PHYSICAL PROPERTIES

2 3 volume of soil in an acre-furrow-slice, therefore, 100,000,000 cm x 15 cm x 1.3 g/cm is 25,410 ft3 to a depth of 7 inches. If the soil has a = 1,950,000,000 g 3 bulk density of 1.3 g/cm , the soil has a density The weight of a soil layer for a hectare that is 15 1.3 times that of water. Because a cubic foot of centimeters thick is, therefore, 1,950,000 kg/ha. water weighs 62.4 pounds, a cubic foot of the soil would weigh 81.1 (62.4 x 1.3) pounds. The weight of a 7-inch-thick acre-furrow-slice of a soil SOIL PORE SPACE AND POROSITY with a bulk density of 1.3 g/cm3 (81 lb/ft3 ) is calculated as follows: The fact that mineral soils have a particle density and bulk density of about 2.65 and 1.3 g/cm3 , 2 3 1 7/12 ft x 43 560 ft ] x [81.1 lb/ft ] respectively, means that soils have about 50 per- = 2,061,259 lb cent total pore space or porosity. In simple terms, This value-2,061,259-is usually rounded off as rocks without pore space are broken down by 2,000,000 pounds as the weight of an acre-furrow- weathering to form mineral soils that have about slice of typical oven-dry loamy soil. 50 percent porosity. The pore spaces vary in size, Soil samples to assess the fertility of a soil are and the size of pore spaces themselves can be as i usually taken to the depth of the plow layer. The mportant as the total amount of pore space. results are commonly reported as pounds of potassium, for example, per acre. Because it is usually assumed that the plow layer of an acre weighs Determination of Porosity 2 million pounds, the test results are sometimes Oven dry soil cores, which have been used to reported as parts per 2 million (pp2m). For exam- determine bulk density, can be placed in a pan of ple, a soil test result for available potassium of 100 water, allowed to saturate or satiate, and then pounds per acre (acre-furrow-slice) is sometimes reweighed to obtain the data needed to calculate given as 100 pp2m. In recent years there has been soil porosity. When the soil is satiated, the pore a tendency to plow deeper and, in many cases, the space is filled with water, except for a small furrow slice is considered to be 9 or 10 inches amount of entrapped air. The volume of water in a thick and to weigh proportionally more. satiated soil core approximates the amount of pore space and is used to calculate the soil's PROBLEM: Calculate the weight of a 10-inch- approximate porosity. For example, if the soil in a thick acre-furrow-slice that has a bulk density of 400 cm2 core weighed 600 grams at oven dry, and 1.2 g/cm 3 . 800 grams at water satiation, the satiated soil would contain 200 grams of water that occupies [ 10/12 ft x 43 560 ft'] x [74.9 lb/ft 3 ] 200 cubic centimeters of space (1 g of water has a = 2,718,870 lb volume of 1 cm3 ). Porosity is calculated as Lime recommendations for increasing soil pH are follows: based on assumptions in regard to the weight of cm3 pore space the soil volume that must be limed. porosity = 3 X 100 cm soil volume 200 cm3 Soil Weight on a Hectare Basis x 100 = 50% A hectare is an area 100 meters square and con- 400 cm3 tains 10,000 square meters (m2) or 100,000,000 cm 2 . A 15 centimeter thick layer of soil over a PROBLEM: A 500 cm3 oven dry core has a bulk hectare that has a bulk density of 1.3 g/cm 3 would density of 1.1 g/cm3 . The soil core is placed in a weigh pan of water and becomes water saturated. The SOIL PORE SPACE AND POROSITY 35 oven dry soil and water at saturation weigh 825 It has been pointed out that sand surface soils grams. Calculate the total soil porosity. have less porosity than clayey surface soils. Yet, our everyday experiences tell us that water moves much more rapidly through the sandy soil. The explanation of this apparent paradox lies in the pore size differences in the two soils. Sands contain mostly macropores that normally cannot retain water against gravity and are usually filled with air. As a consequence, macropores have also been called aeration pores. Since the porosity of sands is composed mainly of macropores, sands transmit water rapidly. Pores that are small Effects of Teture and Structure enough to retain water against gravity will remain on Porosit water filled after soil wetting by rain or irrigation, Spheres in closest packing result in a porosity of and are called capillar or micropores. 26 percent, and spheres in open packing have a Because sands have little micropore space they porosity of 48 percent. Stated in another way, a are unable to retain much water. Fine-textured ball that just fits in a box occupies 52 percent of soils tend to contain mainly micropores and thus the volume of the box, whereas 48 percent of the are able to retain a lot of water but have little volume is empty space. These facts are true re- ability to transmit water rapidly. An example of the gardless of size of the spheres or balls. Single- distribution of micropore and macropore space grained sands have a porosity of about 40 percent. in the various horizons of a soil profile is given in This suggests that the sand particles are not per- Figure 3.10. Note that the amounts of both total fect spheres and the sand particles are not in a porosity and macropore space are inversely re- perfect close packing arrangement. The low po- lated to the bulk density. rosity of single-grained sands is related to the absence of structure (peds) and, therefore, an absence of interped spaces. Porosit and Soil Aeration Fine-textured A horizons, or surface soils, have The atmosphere contains by volume nearly 79 a wide range of particle sizes and shapes, and the percent nitrogen, 21 percent oxygen, and 0.03 particles are usually arranged into peds. This percent carbon dioxide. Respiration of roots, and results in pore spaces within and between other organisms, consumes oxygen and produces peds. These A horizons with well-developed gran- carbon dioxide. As a result, soil air commonly ular structure may have as much as 60 per- contains 10 to 100 times more carbon dioxide and cent porosity and bulk density values as low as slightly less oxygen than does the atmosphere 1.0 g/cm3 . Fine-textured Bt horizons have a differ- (nitrogen remains about constant). Differences in ent structural condition and tend to have less the pressures of the two gases are created beween porosity and, consequently, a greater bulk density the soil and atmosphere. This causes carbon than fine-textured A horizons. This is consistent dioxide to diffuse out of the soil and oxygen to with the filling of pore space by translocated clay diffuse into the soil. Normally, this diffusion is and the effects of the weight of the overlying soil, sufficiently rapid to prevent an oxygen deficiency which applies a pressure on the Bt horizon. The or a carbon dioxide toxicity for roots. pore spaces within the peds will generally be Although water movement through a uniformly smaller than the pore spaces between the peds, porous medium is greatly dependent on pore size, resulting in a wide range in pore sizes. the movement and diffusion of gas are closely 36 SOIL PHYSICAL PROPERTIES correlated with total porosity. Gaseous diffusion susceptible to oxygen deficiency and may be in soil, however, is also dependent on pore space killed when soils are water saturated for a few continuity. When oxygen is diffusing through a hours (less than a day). Two yew plants that were macropore and encounters a micropore that is planted along the side of a house at the same time filled with water, the water-filled micropore acts are shown in Figure 3.11. The plant on the right as a barrier to further gas movement. Diffusion of side died from oxygen stress caused by water- oxygen through the water barrier is essentially saturated soil as a result of flooding from the zero because oxygen diffusion through water is downspout water. The soil had a high clay con- about 10,000 times slower than through air. tent. The use of pipes to carry water away from Clayey soils are particularly susceptible to poor building foundations is also important in prevent- soil aeration when wet, because most of the pore ing building foundation damage, where soils with space consists of micropores that may be filled high clay content expand and contract because of with water. Sands tend to have good aeration or cycles of wetting and drying. gaseous diffusion because most of the porosity is When changes in soil aeration occur slowly, composed of macropores. In general, a desirable plants are able to make some adjustment. All soil for plant growth has a total porosity of 50 plant roots must have oxygen for respiration, and percent, which is one half macropore porosity plants growing on flooded soil or in swamps have and one half micropore porosity. Such a soil has a special mechanisms for obtaining oxygen. good balance between the retention of water for plant use and an oxygen supply for root respiration. SOIL COLOR Oxygen deficiencies are created when soils become water saturated or satiated. The wilting of Color is about the most obvious and easily deter- tomato, due to water-saturated soil, is shown in mined soil property. Soil color is important be- Figure 1.3. Plants vary in their susceptibilty to cause it is an indirect measure of other important oxygen deficiency. Tomatoes and peas are very characteristics such as water drainage, aeration,

FIGURE 3.11 Both of these yews were planted at the same time along the side of a new house. The failure to install a pipe on the right, to carry away the downspout water, caused water-saturated soil and killed the plant. SOIL COLOR 37

and the organic matter content. Thus, color is considerable decrease in organic matter content used with other characteristics to make many im- with increasing soil depth. In some soils, finely portant inferences regarding soil formation and divided manganese oxides contribute to black land use. color. The major coloring agents of most subsoil horizons are iron compounds in various states of oxi- Determination of Soil Color dation and hydration. The rusting of iron is an Soil colors are determined by matching the color oxidation process that produces a rusty or of a soil sample with color chips in a Munsell reddish-colored iron oxide. The bright red color soil-color book. The book consists of pages, each of many tropical soils is due to dehydrated and having color chips arranged systematically ac- oxidized iron oxide, hematite (Fe2O3). Organic cording to their hue, value, and chroma, the three matter accumulation in the A horizons of these variables that combine to give colors. Hue refers soils results in a brownish-red or mahogany color. to the dominant wavelength, or color of the light. Hydrated and oxidized iron, goethite (FeOOH), Value, sometimes color brillance, refers to the has a yellow or yellowish-brown color, and re- quantity of light. It increases from dark to light duced and hydrated iron oxide has a gray color. colors. Chroma is the relative purity of the domi- Various combinations of iron oxides result in nant wavelength of the light. The three properties brown and yellow-brown colors. Thus, the color are always given in the order of hue, value, and of the iron oxides is related to aeration and hydra- chroma. In the notation, 10YR 6/4, 10YR is the tion conditions as controlled by the absence or hue, 6 is the value, and 4 is the chroma. This color presence of water. Soils on slopes that never satu- is a light-yellowish brown. This color system en- rate with water have subsoils indicative of well- ables a person to communicate accurately the drained and aerated soil-subsoils with reddish color of a soil to anyone in the world. and brownish colors. Soils in depressions that collect water, and poorly drained locations in which soils are water saturated much of the time, Factors Affecting Soil Color will tend to have gray-colored B horizons. Soils Organic matter is a major coloring agent that af- in intermediate situations will tend to have fects soil color, depending on its nature, amount, yellowish-colored B horizons. If a soil has a gray- and distribution in the soil profile. Raw peat is colored B horizon, the soil is likely to be water usually brown; well-decomposed organic matter, saturated at least part of the time unless it is such as humus, is black or nearly so. Many or- artificially drained (see Figure 3.12). ganic soils have a black color. The light and grayish colors of E horizons are In most mineral soils, the organic matter con- related to the illuviation of iron oxides and low tent is usually greatest in the surface soil horizons organic matter content. Some soil horizons may and the color becomes darker as the organic mat- have a white color because of soluble salt accu- ter content increases. Even so, many A horizons mulation at the soil surface or calcium carbonate do not have a black color. Black-colored A hori- accumulation in subsoils. Horizons in young soils zons, however, are common in soils that devel- may be strongly influenced by the color of the soil oped under tall grass on the pampa of Argentina parent material. and the prairies of the United States. An interesting color phenomenon occurs in the clayey soils of the Texas Blacklands. The A horizon has a Significance of Soil Color black color; however, the black color may extend Many people have a tendency to equate black- to the depth of a meter even though there is a colored soils with fertile and productive soils. 38 SOIL PHYSICAL PROPERTIES

color are a useful guide in selection of plant species. Alternating water saturation and drying of the subsoil, which may occur because of alternating wet and dry seasons, may produce an intermediate color situation. During the wet season, iron is hydrated and reduced, and during the dry season the iron is dehydrated and oxidized. This causes a mixed pattern of soil colors called mot- tling. Mottled-colored B horizons are indicative of soils that are intermediate between frequent water saturation and soils with continuous well-drained conditions.

SOIL TEMPERATURE Below freezing there is extremely limited biological activity. Water does not move through the soil as a liquid and, unless there is frost heaving, time FIGURE 3.12 Mineral soils that develop under the influence of much soil wetness have a dark-colored stands still for the soil. A soil horizon as cold as A horizon and a gray-colored subsoil. The A horizon 5° C acts as a deterrent to the elongation of roots. is about 40 centimeters thick. The chemical processes and activities of microorganisms are temperature dependent. The alternate freezing and thawing of soils results in the Broad generalizations between soil color and soil alternate expansion and contraction of soils. This fertility are not always valid. Within a local region, affects rock weathering, structure formation, and increases in organic matter content of surface the heaving of plant roots. Thus, temperature is an soils may be related to increases in soil fertility, important soil property. because organic matter is an important reservoir of nitrogen. Subsoil colors are very useful in predicting the Heat Balance of Soils likelihood of subsoil saturation with water and The heat balance of a soil consists of the gains poor aeration. Gray subsoil color indicates a fairly and losses of heat energy. Solar radiation received constant water-saturated condition. Such soils are at the soil surface is partly reflected back into the poor building sites because basements tend to be atmosphere and partly absorbed by the soil sur- wet and septic tank filter fields do not operate face. A dark-colored soil and a light-colored properly in water-saturated soil. Installation of a quartz sand may absorb about 80 and 30 percent drainage system is necessary to use the soil as a of the incoming solar radiation, respectively. Of building site successfully. Subsoils that have the total solar radiation available for the earth, bright brown and red colors are indicative of good about 34 percent is reflected back into space, 19 aeration and drainage. These sites are good loca- percent is absorbed by the atmosphere, and 47 tions for buildings and for the production of tree percent is absorbed by the land. fruits. Many landscape plants have specific needs Heat is lost from the soil by: (1) evaporation of and tolerances for water, and aeration and soil water, (2) radiation back into the atmosphere, SOIL TEMPERATURE 39

(3) heating of the air above the soil, and (4) heat- within the normal rooting depth of trees, whereas ing of the soil. For the most part the gains and soils on southern slopes do not. Trees tend to losses balance each other. But during the daytime grow only on the south-facing slopes without per- or in the summer, the gains exceed the losses, manently frozen subsoils. Considerable variation whereas the reverse is true for nights and winters. in the microenvironment for plants can exist The amount of heat needed to increase the around a building. Southern exposures are temperature of soil is strongly related to water warmer, drier, and have more light than northern content. It takes only 0.2 calories of heat energy to exposures. increase the temperature of I gram of dry soil 1 ° C; Large bodies of water act as heat sinks and compared with 1.0 calorie per gram per degree for buffer temperature changes nearby. In the spring water. This is important in the temperate regions and fall the temperature changes more slowly and where soils become very cold in winter and plant- gradually in the area adjacent to the Great Lakes ing dates in the spring depend on a large rise in than at locations further inland. Near these lakes soil temperature. In general, sandy soils warm in the spring, the temperatures of both air and soil more quickly and allow earlier planting than do increase slowly, which delays the blossoms on fine-textured soils, because sands retain less wa- fruit trees and thereby reduces the hazard of late ter and heat up faster. spring frosts. Killing frosts in the fall are delayed, resulting in an extension of the growing season. As a consequence, production of vine and tree Location and Temperature fruits is concentrated in areas adjacent to the In the northern hemisphere, soils located on Great Lakes. The effect of distance from Lake Erie southern slopes have a higher temperature than on the dates of last killing frost in spring and first soils on north-facing slopes. Soils on south-facing killing frost in the fall is shown in Figure 3.13. slopes are more perpendicular to the sun's rays and absorb more heat energy per unit area than do soils on northern slopes. This is very obvious in Control of Soil Temperature tundra regions where soils on north-facing slopes Two practical steps can be taken to change soil may have permanently frozen subsoil layers temperature. Wet soils can be drained to remove

FIGURE 3.13 Dates for last killing frost in spring and first killing frost in the fall at left and right, respectively, for Erie County, Pennsylvania. The longer growing season along Lake Erie favors land use for fruits and vegetables. (Data from Taylor, 1960.) 40 SOIL PHYSICAL PROPERTIES water, and mulches can be applied on the surface of the soil to alter the energy relationships. When used for agriculture, wet soils are drained to create an aerated root zone. The removal of water also causes the soil to warm more quickly in the spring. A light-colored organic matter mulch, such as straw, will tend to lower soil temperature because (1) more solar radiation will be reflected and less absorbed, and (2) the water content of the soil will tend to be greater because more water will infiltrate. The mulch will, however, tend to increase soil temperature because the mulch tends to: FIGURE 3.14 Soil with permafrost having a layer of (1) reduce loss of heat by radiation, and (2) re- water-saturated soil overlying the permafrost. Even in duce water evaporation from the soil surface, summer, with little precipitation, soils are wet. Frost heaving causes an irregular soil surface with rounded which requires energy. The net effect, however, is hummocks. to reduce soil temperature in the spring. In the northern corn belt, crop residues left on the soil The base of the active layer is the upper surface surface promote cooler soils in the spring and of the permafrost, or the permafrost table. The tend to result in slightly lower corn yields. A black melting of frozen soil ice in summer results in wet plastic mulch increases soil temperature because soil conditions, even if there is little or no rain- it increases absorption of solar radiation, reduces fall, because the permafrost inhibits the down- heat loss by radiation, and reduces evaporation of ward movement of water from the soil. Alternate water from the soil surface. Black mulches have freezing and thawing of wet soil above the perma- been used to increase soil temperatures in order frost produces cryoturbation (movement and mix- to produce vegetables and melons for an earlier market. FIGURE 3.15 The utilidor system for servicing buildings in the Northwest Territories, Canada. Utility lines (water, sewage) in above-ground utilidor are Permafrost protected from breakage resulting from soil When the mean annual soil temperature is below movement associated with freezing and thawing in 0° C, the depth of freezing in winter may exceed soils with permafrost. The utilidor also heats the the depth of thawing in summer. As a conse- water and sewage pipes to prevent freezing. quence, a layer of permanently frozen soil, called permafrost, may develop. Permafrost ranges from material that is essentially all ice to frozen soil, which appears ordinary except that it is frozen and hard. The surface layer that thaws in summer and freezes in winter is the active layer. The active layer is used by plants. Permafrost is widespread in tundra regions and where it occurs there is, generally, an absence of trees. Low-growing shrubs, herbs, and grasses grow on the soils with permafrost if the temperature is above freezing in the summer. REFERENCES 41 ing of soil due to freezing and thawing). Soil wet- with favorable conditions for water retention and ness from melting of ice and the irregular soil aeration, about one half of the porosity is macro- surface (hummocks) due to cryoturbation are pore space and one half is micropore space. shown in Figure 3.14. Excessive soil movement Soil color is used as an indicator of organic necessitates the distribution of utilities to build- matter content, drainage, and aeration. ings above ground with utilidors, as shown in Soil temperature affects plant growth. Soil tem- Figure 3.15. perature is greatly affected by soil color, water When the vegetative cover is disturbed by traffic content, and the presence or absence of surface or building construction, more heat enters the soil materials, such as mulches. Permafrost occurs in and more ice melts. In some cases enough ice soils with average temperature below freezing. melts to cause soil subsidence. Buildings are built on stilts to shade the soil in summer, to promote minimum heat transfer from buildings to REFERENCES soil, and to allow maximum cooling and freezing of soil in winter. Chesters, G. 0., 0. J. Attoe, and 0. N. Allen. 1957. "Soil Aggregation in Relation to Various Soil Constitu- ents." Soil Sci. Soc. Am. Proc. 21:272-277. Day, P. R. 1953. "Experimental Confirmation of Hy- SUMMARY drometer Theory."Soil Sci. 75:181-186. Foth, H. D. 1983. "Permafrost in Soils." J Agron. Educ. Soil physical properties affect virtually every use 12:77-79. made of the soil. Soil Survey Staff. 1951. Soil Surve Manual. U.S.D.A. Texture relates to the amount of sand, silt, and Handbook 18. Washington, D.C. clay in the soil, and structure relates to the ar- Taylor, D. C. 1960. Soil Surve of Erie Count, Pennsl- rangement of the sand, silt, and clay into peds. vania. U.S.D.A. and Pennsylvania State University Texture and structure greatly affect plant growth Tedrow, J. C. F. 1977. Soils of Polar Landscapes. Rut- by influencing water and air relationships. Soils gers University Press, New Brunswick, N.J. Wascher, H. L. 1960. "Characteristics of Soils Associ- that expand and shrink with wetting and drying ated with Glacial Tills in Northeastern Illinois." Univ. affect the stability of building foundations. Illinois Agr. Epp. Sta. But. 665. About one half of the volume of mineral soils is Wilde, S. A. 1935. "The Significance of Texture in For- pore space. Such soils have a bulk density of estry and Its Determination by a Rapid Field Method." about 1.3 g/cm 3 and 50 percent porosity. In soils Jour. For. 33:503-508. CHAPTER 4

TILLAGE AND TRAFFIC

The beginning of agriculture marks the beginning monly accepted purposes of tillage: (1) to man- of soil tillage. Crude sticks were probably the first age crop residues, (2) to kill weeds, and (3) to tillage tools used to establish crops by planting the alter soil structure, especially preparation of the vegetative parts of plants, including stem sec- soil for planting seeds or seedlings. tions, roots, and tubers. Paintings on the walls of ancient Egyptian tombs, dating back 5,000 years, depict oxen yoked together pulling a plow made Management of Crop Residues from a forked tree. By Roman times, tillage tools Crops are generally grown on land that contains and techniques had advanced to the point that the plant residues of a previous crop. The mold- thorough tillage was a recommended practice for board plow is widely used for burying these crop crop production. Childe (1951) considered the residues in humid regions. Fields free of trash discovery and development of the plow to be one permit precise placement of seed and fertilizer at of the 19 most important discoveries or applica- planting and permit easy cultivation of the crop tions of science in the development of civili- during the growing season (see Figure 4.1). zation. Now, paradoxically, the use of the plow for In subhumid and semiarid regions, by contrast, primary tillage is being challenged. the need for wind erosion control and conservation of moisture have led to the development of tillage practices that successfully establish crops EFFECTS OF TILLAGE ON SOILS AND without plowing. The plant residues remaining on PLANT GROWTH the soil surface provide some protection from wa- Tillage is the mechanical manipulation of soil for ter and wind erosion. Plant residues left on the any reason. In agriculture and forestry, tillage is land over winter may trap snow and increase the usually restricted to the modification of soil con- water content of soils when the trapped snow ditions for plant growth. There are three com- melts.

EFFECTS OF TILLAGE ON SOILS AND PLANT GROWTH 43

disintegration or breakdown remains unchanged. Consequently, tillage with cultivators, disks, and packers crushes some of the soil peds and tends to reduce soil porosity. Exposed cultivated land suffers from disruption of peds by raindrop impact in the absence of a vegetative cover. In addition, tillage hastens organic matter decomposition. Therefore, the long-term effect of plowing and cultivation is a more compacted soil as a result of the crushing of peds and subsequent FIGURE 4.1 Tillage (disking) in the spring to settling of soil. prepare the land for corn planting where the land was plowed the previous fall and virtually all of the When forest or grassland soils are converted to previous crop residues were buried. This type of use for crop production, there is a decline in soil tillage allows planting and cultivating without aggregation and soils become more compact be- interference from plant residues, but leaves the soil cause of the long-term effects of tillage. Compac- bare and susceptible to water runoff and soil erosion. tion results in a decrease in the total pore space Such tillage is representative of conventional tillage systems. and an increase in the bulk density. Pushing particles together as a result of tillage results in a decrease in the average pore size. Some of the macropores are reduced to micropores, and thus Tillage and Weed Control there is an increase in the volume of micropore Weeds compete with crop plants for nutrients, space and a decrease in the amount of macropore water, and light. Weeds, however, can be con- space. The overall decrease in total porosity re- trolled with herbicides. If weeds are eliminated sults from a greater decrease in macropore space without tillage, can cultivation of row crops be than the increase in micropore space. An increase eliminated? Data from many experiments support in micropore space, or water-filled space, be- the conclusion that the major benefit of culti- cause of the compaction of clayey soils, is detri- vating corn is weed control. Cultivation late in the mental because of reduced aeration and water growing season may be detrimental, because movement. By contrast, the compaction of a sand roots may be pruned and yields reduced. Culti- soil could be desirable because the soil could vation to control weeds is important in sub- then retain more water and still be sufficiently humid regions to increase and conserve soil wa- aerated. ter when crops are not growing during a fallow A study was made of the effects of growing period. cotton for 90 years on Houston Black Clay soils in the U.S. Texas Blacklands. Soil samples from cultivated fields were compared with samples ob- Effects of Tillage on Structure tained from an adjacent area that had remained and Porosity uncultivated in grass. Long-time cultivation All tillage operations change the structure of the caused a significant decrease in soil aggregation soil. The lifting, twisting, and turning action of and total porosity and an increase in bulk density. a moldboard plow leaves the soil in a more ag- Changes in total porosity inversely paralleled the gregated condition. Cultivation of a field to kill changes in bulk density. These changes are weeds may also have the immediate effect of shown in Figure 4.2. The most striking and impor- loosening the soil and increasing water infil- tant change was the decrease in macropore tration and aeration. The resistance of peds to space, which was reduced to about one half of 44 TILLAGE AND TRAFFIC

FIGURE 4.2 Effects of 90 years of cultivation on pore

space³) and bulk density (gm/ CM of Houston Black Clay soils. Macropore and total pore space decreased and the micropore space increased. Decreases in total porosity resulted in increases in bulk density. (Data from Laws and Evans, 1949.) that of the uncultivated soil. Note that these grow well in experiments without tillage, the pro- changes occurred to a depth of at least 75 cen- duction of most crops will generally require some timeters. tillage. Sugar beet yields were lowest when the land was not worked or tilled at all and when the land was worked four or more times, between Surface Soil Crusts plowing and planting. The highest yields occurred During crop production, the surface of the soil is where the land was worked or tilled only once or exposed and left bare for varying periods of time twice between plowing and planting, as shown in without the protection of vegetation or crop resi- Table 4.1. The data in Table 4.1 show that exces- dues. Soil peds at the soil surface are broken apart sive tillage is detrimental. Some tillage is neces- by raindrops, and the primary particles (sand, silt, sary for the practical production of some crops; clay) are dispersed. The splashing raindrops and however, the total amount of tillage used is de- the presence of water cause the smallest particles clining. The result has been considerable pro- to move into the spaces between the larger particles. A thin, dense-surface crust may form and reduce the water infiltration rate by a thousand- TABLE 4.1 Sugar Beet Yields as Affected by the fold or more. Thus, surface crust formation in- Number of Times a Field Was Worked Prior creases water runoff and erosion on sloping land. to Planting When the surface crust dries, the crust may become very hard and massive, inhibiting seedling emergence. Crust hardness increases with increased clay content of the soil and the extent of soil drying.

Minimum and Zero Tillage Concepts It is obvious that plants in natural areas grow without tillage of the soil. Even though plants Adapted from Cook, et al., 1959. EFFECTS OF TILLAGE ON SOILS AND PLANT GROWTH 45 gress in the development of minimum tillage and In no-till systems for row crop production, a no-till tillage systems. narrow slot is made in the untilled soil so that Minimum tillage is the minimum soil manipu- seed can be planted where moisture is adequate lation necessary for crop production under the for germination. Weed control is by herbicides. existing soil and climatic conditions. Some of the The residues of the previous crop remain on the minimum tillage equipment prepares the land for soil surface, which results in greatly reduced wa- planting, plants the seeds, and applies fertilizer ter runoff and soil erosion. There was 4,000 and herbicide in one trip over the field. Under pounds per acre of wheat residue on the field favorable conditions, no further tillage will be re- when the soybeans were planted in the field quired during the growing season. Press wheels shown in Figure 4.4. The coverage of the soil behind the planter shoes pack soil only where the surface with crop residues with various tillage seed is placed, leaving most of the soil surface in systems is shown in Table 4.2. Tillage systems a state conducive to water infiltration. Experimen- with reduced tillage and those where crop resi- tal evidence shows that minimum tillage reduces dues are left on the soil surface minimize water water erosion on sloping land. It also makes weed runoff and soil erosion; this is called coneaion control easier because planting immediately fol- illage. lows plowing. Weed seeds left in loose soil are at From this discussion of tillage, it should be a disadvantage because the seeds have minimum obvious that tillage is not a requirement of plant contact with soil for absorbing water. Crop yields growth. Generally, gardens do not need to be are similar to those where more tillage is used, but plowed. The only tillage necessary is that needed the use of minimum tillage can reduce energy use for the establishment of the plants and the con- and overall production costs. Some of the sav- trol of weeds. Mulches in gardens, like crop resi- ings, conversely, may be used to pay for herbicide dues in fields, protect the soil from raindrop costs. impact and promote desirable soil structure. No-till is a procedure whereby a crop is planted A major benefit is a greater water- infitration directly into the soil with no preparatory tillage rate and a shorter amount of time needed to since the harvest of the previous crop. The plant- irrigate. ing of wheat in the wheat stubble from the previous crop without any prior tillage (such as plowing) is shown in Figure 4.3. Tilth and Tillage Tilth is the physical condition of the soil as related to the ease of tillage, fitness of soil as a seedbed, FIGURE 4.3 No-till planting of wheat in the plant residues of the previous year's wheat crop. and desirability for seedling emergence and root (Photograph courtesy Soil Conservation Service, penetration. Tilth is related to structural condi- USDA.) tions, the presence or absence of impermeable layers, and the moisture and air content of the soil. The effect of tillage on tilth is strongly related to soil water content at the time of tillage. This is especially true when tillage of wet clay soils creates large chunks or clods that are difficult to break down into a good seedbed when the chunks dry. As shown earlier, cropping usually causes aggregate deterioration and reduced porosity. Farm managers need to exercise considerable judgment concerning both the kind of tillage and the timing of tillage operations to minimize the FIGURE 4.4 Soybeans planted no-till in the residues of a previous wheat crop. The previous crop left about 4,000 pounds of wheat residue per acre. (Photograph courtesy Soil Conservation Service, USDA.)

detrimental effects and to maximize the beneficial soil compaction. These changes in porosity are effects of tillage on soil tilth. important enough to warrant their summarization as an aid to a further discussion of the effects of traffic on soils. Soil compaction results in: (1) a TRAFFIC AND SOIL COMPACTION decrease in total pore space, (2) a decrease in macropore space, and (3) an increase in micro- The changes in soil porosity, owing to compac- pore space. tion, are caused by the long-term effects of cropping and tillage and by the pressure of tractor tires, animal hooves, and shoes. As tractors and machinery get larger, there is greater potential for Compaction Layers Compaction at the bottom of a plow furrow frequently occurs during plowing because of tractor TABLE 4.2 Effect of Tillage System on Amount of wheels running in a furrow that was made by the Residue Cover in Continuous Corn Immediately previous trip over the field. Subsequent tillage is After Planting usually too shallow to break up the compacted soil, and compaction gradually increases. This type of compaction produces a layer with high bulk density and reduced porosity at the bottom of the plow layer, and it is appropriately termed a plow pan, or pressure pan. Pressure pans are a problem on sandy soils that have insufficient clay content to cause enough swelling and shrinking, via wetting and drying, to naturally break up the compacted soil. Bulk densities as high as TRAFFIC AND SOIL COMPACTION 47

1.9 g/cm ³ have been found in plow pans of sandy Potatoes are traditionally grown on sandy soils coastal plain soils in the southeastern United that permit easy development of well-shaped States. Cotton root extension was inhibited in tubers. Resistance to tuber enlargement in com- these soils when the bulk density exceeded pacted soils not only reduces tuber yield but in- 1.6 g/cm3 . The effect of a compact soil zone on creases the amount of deformed tubers, which the growth of bean roots is shown in Figure 4.5. have greatly reduced market value. In studies where bulk density was used as a measure of soil compaction, potato tuber yield was negatively Effects of Wheel Traffic on Soils correlated and the amount of deformed tubers and Crops was positively correlated with bulk density. As much as 75 percent of the entire area of an alfalfa field may be run over by machinery wheels in a single harvest operation. The potential for Effects of Recreational Traffic plant injury and soil compaction is great, because Maintenance of a plant cover in recreational areas there are 10 to 12 harvests annually in areas where is necessary to preserve the natural beauty and to alfalfa is grown year-round with irrigation. Wheel prevent the undesirable consequences of water traffic damages plant crowns, causing plants to runoff and soil erosion. Three campsites in the become weakened and more susceptible to dis- Montane zone of Rocky Mountain National Park in ease infection. Root development is restricted. Colorado were studied to determine the effects of Alfalfa stands (plant density) and yields have camping activities on soil properties. Soils in been reduced by wheel traffic. areas where tents were pitched, and where fire- places and tables were located, had a bulk density ³ ³ FIGURE 4.5 Bean root growth in compacted soil at of 1.60 g/cm compared with 1.03 g/cm in areas 4-inch depth on left and no soil compaction on the with little use. The results show that camping right. activities can compact soil as much as tractors and other heavy machinery. Snowmobile traffic on alfalfa fields significantly reduced forage growth at one out of four locations in a Wisconsin study. It appears that injury to alfalfa plants and reduced yields are related to snow depth. There is less plant injury as snow depth increases. The large surface area of the snowmobile treads makes it likely that soil compaction will be minimal or nonexistent. Moderate traffic on fields with a good snow cover appears to have little effect on soil properties or dormant plants buried in the snow. Human traffic compacts soil on golf courses and lawns. Aeration and water infiltration can be increased by using coring aerators. Numerous small cores about 2 centimeters in diameter and 10 centimeters long are removed by a revolving drum and left lying on the ground, as shown in Figure 4.6. One of the most conspicuous recent changes in 48 TILLAGE AND TRAFFIC some of the arid landscapes of the southwestern disturb or break shallow tree roots that are grow- United States has been caused by use of all-terrain ing just under the surface soil layer. vehicles. In the Mojave Desert one pass of a mo- The ease with which water can flow through the torcycle increases bulk density of loamy sand soil was found to be 65 and 8 percent as great on soils from 1.52 to 1.60 g/cm 3 , and 10 passes in- cutover areas and on logging roads, respectively, crease the bulk density to 1.68 g/cm 3 , according in comparison with undisturbed areas in south- to one study. Parallel changes in porosity oc- western Washington. The reductions in perme- curred with the greatest reduction in the largest ability increase water runoff and soil erosion, re- pores. Water infiltration was decreased and water sulting in less water being available for the growth runoff and soil erosion were greatly accelerated. of tree seedlings or any remaining trees. Reduced Recovery of natural vegetation and restoration of growth and survival of chlorotic Douglas-fir seed- soils occur very slowly in deserts. Estimates in- lings on tractor roads in western Oregon was con- dicate that about 100 years will be required to sidered to be caused by poor soil aeration and low restore bulk density, porosity, and infiltration nitrogen supply. capacity to their original values based on extrapo- Changes in bulk density and pore space of for- lation of 51 years of data from an abandoned town est soils, because of logging, are similar to in southern Nevada. changes produced by longtime cropping. After logging, and in the absence of further traffic, however, forest soils are naturally restored to their Effects of Logging Traffic on Soils and former condition. Extrapolation of 5 years' data Tree Growth suggests that restoration of logging trails takes 8 The use of tractors to skid logs has increased years and wheel ruts require 12 years in northern because of their maneuverability, speed, and Mississippi. In Oregon, however, soil compaction economy. About 20 to 30 percent of the forest land in tractor skid trails was still readily observable may be affected by logging. Tractor traffic can after 16 years.

FIGURE 4.6 Soil aerator used to increase soil aeration on a college campus in California. The insert shows detail of removal of small cores that are left lying on the surface of the lawn. TRAFFIC AND SOIL COMPACTION 49

Controlled Traffic bulk density between a regular tillage system The pervasive detrimental effects of soil compac- using a moldboard plow and a conservation (re- tion have caused great interest in tillage research duced) tillage system with normal trafficking of (see Figure 4.7). The extensive coverage of the machinery wheels. When the tillage was carried soil surface by machinery wheels, producing soil out with the machinery wheels running on the compaction, has resulted in the development of same path, no-wheel or controlled traffic, com- controlled traffic tillage systems. Controlled traffic paction caused by wheel traffic was eliminated systems have all tillage operations performed in and bulk densities were lower in both moldboard fixed paths so that recompaction of soil by traffic plow and conservation tillage than for wheel traf- (wheels) does not occur outside the selected fic. In the no-wheel traffic experiments, the con- paths. In fields with controlled traffic, only the servation tillage treatment eventually showed small amount of soil in the wheel tracks is com- lower bulk densities (less compaction) than pacted, and this compacted soil has almost no the moldboard plow treatment, as shown in effect on crop yields. One additional benefit of Figure 4.8. controlled traffic is that the machines run on hard A controlled traffic experiment on sandy loam and firm paths that permit the planting of crops in soil in California showed that alfalfa yields with soils with much greater water content than nor- normal traffic were 10 percent less as compared mal. Tractors do not get stuck and crops can be to controlled traffic. The lower yields were also planted at an earlier date. associated with more compact soils having An experiment was conducted in Minnesota to greater bulk density. An experimental controlled study the effects of machinery wheels on soil traffic machine for planting small grain is shown compaction in two tillage systems. The 9-year ex- in Figure 4.9. periment showed there was no difference in soil In instances where repeated traffic is necessary,

FIGURE 4.7 Tillage research facilities at the USDA National Soil Dynamics Laboratory in Auburn, Alabama. (Photograph courtesy of James H. Taylor.) 50 TILLAGE AND TRAFFIC

percent of the rice is grown in Asia, mostly in small fields or paddies that have been leveled and bunded (enclosed with a ridge to retain water). The paddy fields are flooded and tilled before rice transplanting. A major reason for the tillage is the destruction of soil structure and formation of soil that is dense and impermeable to water. The practice permits much land that is normally permeable to water to become impermeable and suited for wetland rice production. In the United States, naturally water-impermeable soils are used for rice production and the soils are not puddled. A small amount of upland rice is grown, like wheat, on unflooded soils. FIGURE 4.8 Bulk density of the 0- to 15-centimeter soil layer as affected by tillage and wheel traffic over time. (Reproduced from Soil Science Society America Journal, Vol. 48, No. 1, January-February 1984, p. Effects of Flooding 152-156 by permission of the Soil Science Society of Flooding of dry soil causes water to enter peds America.) and to compress the air in the pores, resulting in small explosions that break the peds apart. The it is generally best to use the same tracks because anaerobic conditions (low oxygen supply) from most of the compaction occurs with the first pass prolonged flooding cause the reduction and dis- over the soil. solution of iron and manganese compounds and the decomposition of organic structural-binding materials. Ped or aggregate stability is greatly re- FLOODING AND PUDDLING OF SOIL duced and the aggregates are easily crushed. The Rice or paddy (wetland rice) is one of the world's disruption of soil peds and the clogging of pores three most important food crops: more than 90 with microbial wastes reduce soil permeability.

ncorporation of wheat seed at a depth FIGURE 4.9 I Effects of Puddling of 5 centimeters in a wet soil in a controlled traffic system where machinery wheels constantly run in the Puddling is the tillage of water-saturated soil same tracks. (Photograph courtesy National Soil when water is standing on the field, as shown in Dynamics Laboratory, USDA.) Figure 4.10. Peds that are already weakened by flooding are worked into a uniform mud, which becomes a two-phase system of solids and liquids. Human or animal foot traffic is effective in forming a compact layer, or pressure pan, about 5 to 10 centimeters thick at the base of the puddled layer. The development of a pressure pan allows for the conversion of soils with a wide range in texture and water permeability to become impermeable enough for rice production. Puddling increases bulk density, eliminates large pores, and increases capillary porosity in the soil above the FLOODING AND PUDDLING OF SOIL 5 1

FIGURE 4.10 Puddling soil to prepare land for paddy (rice) transplanting.

pressure pan. Soil stratification may occur in soils Oxygen Relationships in Flooded Soils when sand settles before the silt and clay. This The water standing on a flooded soil has an oxy- results in the formation of a thin surface layer gen content that tends toward equilibrium with enriched with silt and clay, also having low per- the oxygen in the atmosphere. This oxygenated meability. These changes are desirable for rice water layer supplies oxygen via diffusion to a thin production because soil permeability is reduced upper layer of soil about a centimeter thick, as by a factor of about 1,000 and the amount of water shown in Figure 4.11. This thin upper layer retains needed to keep the field flooded is greatly re- the brownish or reddish color of oxidized soil. duced. Much of the rice is grown without a de- The soil below the thin surface layer and above pendable source of irrigation water, and rainfall the pressure pan is oxygen deficient, and reduc- can be erratic. Low soil permeability is, therefore, ing conditions exist. Colors indicative of reduced of great importance in maintaining standing water iron and manganese occur, that is, gray and black on the paddy during the growing season. Standing colors. The soil in the immediate vicinity of rice water promotes rice growth (because of no water roots is oxidized because of the transport of oxy- stress and an increased supply of nutrients), and gen into the root zone from the atmosphere. This the nearly zero level of soil oxygen inhibits growth produces a thin layer of yellowish soil around the of many weeds. roots where reduced (ferrous) iron has been oxi- Paddies are drained and allowed to dry before dized to ferric iron and precipitated at the root harvest. Where winters are dry and no irrigation surfaces. water is available, a dryland crop is frequently The soil below the pressure pan can be either planted. Drying of puddled soil promotes ped for- reduced or oxidized. If the soil occurs in a depres- mation; however, large clods often form that are sion and is naturally poorly drained, a gray sub- difficult to work into a good seedbed. Pressure soil that is indicative of reduced soil is likely. pans are retained from year to year and create a Many rice paddies are formed high on the land- shallow root zone that severely limits the supply scape, and they naturally have well-aerated sub- of water and nutrients for dry season crops. soils below pressure pans that may retain their 52 TILLAGE AND TRAFFIC

All soil compaction results in higher bulk density and lower total porosity. The volume of macropores is greatly reduced with an accompanying increase in micropores. Soil layers with a bulk density greater than about 1.6 to 1.8 g/cm ³ are barriers for root penetration. Pressure pans, or plow soles, tend to develop at the bottom of plow furrows, especially in sandy soils with minimal capacity for expansion and contraction due to wetting and drying. Minimum and no-till tillage systems have been developed to minimize the effects of tillage and traffic on soil compaction. Only the minimum amount of tillage necessary for the crop and climatic conditions is used in the first method. In no-till, crops are planted directly in the residues of the previous crop with no prior tillage. For row crops, a slit is made in the soil in which the seed is planted. Minimum and no-till tillage leave more residues on the soil surface than conventional tillage, resulting in reduced water runoff and soil erosion, thus they are considered conservation tillage. In Asia, many soils are flooded and puddled to create a soil with minimal water permeability for flooded rice production. A pressure pan is created below a puddled soil layer (root zone).

FIGURE 4.11 Oxidized and reduced zones in flooded paddy soil. REFERENCES bright oxidized colors after paddies are estab- Childe, V. E. 1951. Man Makes Himself. Mentor, New lished. York. Cook, R. L., J. F. Davis, and M. G. Frakes. 1959. "An Analysis of Production Practices of Sugar Beet Farm- ers in Michigan." Mi. Ag. Ep. Sa. Qa. Bl. SUMMARY 42:401-420. Dickerson, B. P. 1976. "Soil Changes Resulting from Tillage is the mechanical manipulation of soil to Tree-Length Skidding." Soil Sci. Soc. Am. Proc. modify soil conditions for plant growth. Tillage is 40:965-966. beneficial for preparing land for planting, control- Dotzenko, A. D., N. T. Papamichos, and D. S. Romine. ling weeds, and managing crop residues. Tillage 1967. "Effect of Recreational Use on Soil and Mois- involves mechanical manipulation of soil and ve- ture Conditions in Rocky Mountain National Park." hicular or animal traffic, which results in soil J. Soil and Water Con. 22:196-197. compaction. Froehlich, H. A. 1979. "Soil Compaction from Logging REFERENCES 53

Equipment: Effects on Growth of Young Ponderosa Phillips, R. E., R. L. Blevins, G. W. Thomas, W. R. Frye, Pine." J. Soil and Water Con. 34:276-278. and S. H. Phillips. 1980. "No-Tillage Agriculture." Griffith, D. R., J. V. Mannering, and W. C. Moldenhauer. Science. 208:1108-1113. 1977. "Conservation Tillage in the Eastern Corn Belt." Steinbrenner, E. C. and S. P. Gessel. 1955. "The Effect of J. Soil and Water Con. 32:20-28. Tractor Logging on Physical Properties of Some For- Grimes, D. W. and J. C. Bishop. 1971. "The Influence est Soils in Southwestern Washington." Soil Sci. Soc. of Some Soil Physical Properties on Potato Yields A. Proc. 19:372-376. and Grade Distribution." Am. Potato 1 48:414- Taylor, J. H. 1986. "Controlled Traffic: A Soil Compac- 422. tion Management Concept." SAE Tech. Paper Series Iverson, R. M., B. S. Hinckley, and R. M. Webb. 1981. 860731. Soc. Auto. Eng. Inc., Warrendale, Pa. "Physical Effects of Vehicular Disturbances on Arid Voorhees, W. B. and M. J. Lindstrom. 1984. "Long-Term Landscapes." Science. 212:915-916. Effects of Tillage Method on Soil Tilth Independent of Laws, W. D. and D. D. Evans. 1949. "The Effects of Wheel Traffic Compaction." Soil Sci. Soc. Am. J. Long-Time Cutltivation on Some Physical and Chemi- 48:152-156. cal Properties of Two Rendzina Soils." Soil Sci. Soc. Walejko, R. N., J. W. Pendleton, W. H. Paulson, R. E. Am. Proc. 14:15-19. Rand, G. H. Tenpas, and D. A. Schlough. 1973. Meek, B. D., E. A. Rechel, L. M. Carter, and W. R. DeTar. "Effect of Snowmobile Traffic on Alfalfa." J. Soil and 1988. "Soil Compaction and its Effect on Alfalfa in Water Con. 28:272-273. Zone Production Systems." Soil Sci. Soc. Am. J. Youngberg, C. T. 1959. "The Influence of Soil Condi- 52:232-236. tions Following Tractor Logging on the Growth of Moormann, F. R. and N. van Breemen. 1978. Rice: Soil, Planted Douglas-Fir Seedlings." Soil Sci. Soc. Am. Water and Land. Int. Rice Res. Inst., Manila. Proc. 23:76-78. CHAPTER 5

SOIL WATER

Water is the most common substance on the SOIL WATER ENERGY CONTINUUM earth; it is necessary for all life. The supply of fresh water on a sustained basis is equal to As water cascades over a dam, the potential the annual precipitation, which averages 66 cen- energy (ability of the water to do work) decreases. timeters for the world's land surface. The soil, If water that has gone over a dam is returned to the located at the atmosphere-lithosphere interface, reservoir, work will be required to lift the water plays an important role in determining the back up into the reservoir, and the energy content amount of precipitation that runs off the land and of the water will be restored. Therefore, in a ca- the amount that enters the soil for storage and scading waterfall, the water at the top has the future use. Approximately 70 percent of the pre- greatest energy and water at the bottom has the cipitation in the United States is evaporated from lowest energy. As water cascades down a falls, the plants and soils and returned to the atmosphere continuous decrease in energy results in an as vapor, with the soil playing a key role energy continuum from the top to the bottom of in water retention and storage. The remaining the falls. As an analogy, when wet soil dries, there 30 percent of the precipitation represents the is a continuous decrease in the energy content of longtime annual supply of fresh water for the remaining water. When dry soil gets wetter, use in homes, industry, and irrigated agriculture. there is a continuous increase in the energy of soil This chapter contains important concepts and water. The continuous nature of the changes in principles that are essential for gaining an under- the amount of soil water, and corresponding standing of the soil's role in the hydrologic cycle changes in energy, produce the soil water energy and the intelligent management of water re- continuum. sources.

54 SOIL WATER ENERGY CONTINUUM 55

Adheion Wae Water molecules (H 2 0) are electrically neutral; however, the electrical charge within the molecule is asymmetrically distributed. As a result, water molecules are strongly polar and attract each other through H bonding. Soil particles have sites that are both electrically negative and positive. For example, many oxygen atoms are exposed on the faces of soil particles. These oxygen atoms are sites of negativity that attract the positive poles of water molecules. The mutual attraction between water molecules and the surfaces of soil particles results in strong adhesive forces. If a drop of water falls onto some oven dry soil, the FIGURE 5.1 Schematic drawing of relationship water molecules encountering soil particles will between various forms of soil water. Adhesion water be strongly adsorbed and the molecules will is strongly absorbed and very immobile and spread themselves over the surfaces of the soil unavailable to plants. The gravitational water, by particles to form a thin film of water. This layer, or contrast, is beyond the sphere of cohesive forces, and gravity causes gravitational water to flow rapidly film of water, which will be several water mole- down and out of the soil (unless downward flow is cules thick, is called adhesion water (see Figure inhibited). The cohesion water is intermediate in 5.1). properties and is the most important for plant Adhesion water is so strongly adsorbed that it growth. moves little, if at all, and some scientists believe that the innermost layer of water molecules exists in a crystalline state similar to the structure of ice. Adhesion water is always present in field soils and hesive forces operate between water molecules on dust particles in the air, but the water can be because of hydrogen bonding. This causes water removed by drying the soil in an oven. Adhesion molecules to attach themselves to the adhesion water has the lowest energy level, is the most water, resulting in an increase in the thickness of immobile water in the soil, and is generally un- the water film. Water that is retained in soils be- available for use by plant roots and microor- cause of cohesive forces is called cohesion water, ganisms. and is shown in Figure 5.1. About 15 to 20 molecular layers is the maximum amount of water that is normally adsorbed Coheion Wae to soil particle surfaces. With increasing distance The attractive forces of water molecules at the from the soil particle surface toward the outer surface of soil particles decrease inversely and edge of the water film, the strength of the forces logarithmically with distance from the particle that holds the water decreases and the energy and surface. Thus, the attraction for molecules in the mobility of the water increase. Adhesion water is second layer of water molecules is much less than so strongly held that it is essentially immobile, for molecules in the innermost layer. Beyond the and thus is unavailable to plants. The cohesion sphere of strong attraction of soil particles for water is slightly mobile and generally available to water molecules of the adhesive water layer, co- plants. There is a water-energy gradient across the 56 SOIL WATER water films, and the outermost water molecules Gaiaional Wae have the greatest energy, the greatest tendency to In soils with claypans, a very slowly permeable move, and the greatest availability for plant roots. subsoil layer permits little water, in excess of field As roots absorb water over time, the soil water capacity, to move downward and out of the soil. content decreases, and water films become thin- After a long period of rainfall, water accumulates ner; roots encounter water with decreasing energy above the claypan in these soils. As the rainy content or mobility. At some point, water may period continues, the soil above the claypan satu- move so slowly to roots that plants wilt because of rates from the bottom upward. The entire A hori- a lack of water. If wilted indicator plants are zon or root zone may become water saturated. placed in a humid chamber and fail to recover, the Soil water that exists in aeration pores, and that is soil is at the permanent wilting point (see Figure normally removed by drainage because of the 5.2). This occurs when most of the cohesion wa- force of gravity, is gravitational water. When soils ter has been absorbed. are saturated with water, the soil volume compo- When soil particles are close to, or touch, adja- sition is about 50 percent solids and 50 percent cent particles, water films overlap each other. water. Gravitational water in soil is detrimental Small interstices and pores become water filled. when it creates oxygen deficiency. Gravitational In general, the pores in field soils that are small water is not considered available to plants be- enough to remain water filled, after soil wetting cause it normally drains out of soils within a day and downward drainage of excess water has oc- or two after a soil becomes very wet (see Figure curred, are the capillary or micropores. At this 5.1). point, the soil retains all possible water against The soil in the depression, of Figure 5.3 is water gravity (adhesion plus cohesion water), and the saturated because underlying soil is too slowly soil is considered to be at field capacity. The permeable. Note the very poor growth of corn in aeration or macropores at field capacity are air the soil that remains water saturated after rainfall. filled. A typical loamy surface soil at field capacity Along the right side, the soil is not water satu- has a volume composition of about 50 percent rated. The gravitational water has drained out, the solids, 25 percent water, and 25 percent air. aeration pores are air filled, and the soil is at field capacity. Here, the growth of corn is good. An energy gradient exists for the water within a FIGURE 5.2 Wilted coleus plant on the right. If the macropore, including the gravitational water. plant fails to revive when placed in a humid When water filled macropores are allowed to chamber, the soil is at the permanent wilting point in drain, the water in the center of the macropore has regard to this plant. the most energy and is the most mobile. This water leaves the pore first and at the fastest speed. This is followed by other water molecules closer to the pore edges until all of the gravitational water has drained, leaving the cohesion and adhesion water as a film around the periphery of the macropore.

Smma Saemen 1. The forces that affect the energy level of soil water, and the mobility and availability of wa-

ENERGY AND PRESSURE RELATIONSHIPS 5 7

FIGURE 5.3 Water-saturated soil occurs in the depression where corn growth is poor. The surrounding soil at higher elevation is at field capacity because of recent rain, and corn growth is good owing to high availability of both water and oxygen.

ter to plants, are adhesion, cohesion, and in the water could be used to generate electricity. gravity. The greater the water pressure, the greater the 2. A water-saturated soil contains water with tendency of water to move and do work, and the widely varying energy content, resulting in an greater the energy of the water. energy continuum between the oven dry state The next consideration will be that of water and water saturation. pressure relationships in water-saturated soils, 3. The mobility and availability of water for plant which is analogous to the water pressure relation- use parallel the energy continuum. As dry soil ships in a beaker of water or in any body of water. wets over time, the soil water increases in energy, mobility, and availability for plants. As wet soil dries over time, the energy, mobil- Pee Relaionhip in Saaed Soil The beaker shown in Figure 5.4 has a bottom area ity, and plant availability of the remaining wa- ² ter decreases. assumed to be 100 cm . The water depth in the Plants wilt when soil water becomes so im- beaker is assumed to be 20 centimeters. The vol- 4. ³ mobile that water movement to plant roots is ume of water, then, is 2,000 cm and it weighs too slow to meet the transpiration (evapora- 2,000 grams. The pressure, P, of the water at the tion from leaves) needs of plants. bottom of the beaker is equal to force 2,000 g P = = = 20 /cm ² area 100 cm² g ENERGY AND PRESSURE RELATIONSHIPS The water pressure at the bottom of the beaker There is a relationship between the energy of wa- could also be expressed simply as equal to a ter and the pressure of water. Because it is much column of water 20 centimeters high. This is anal- easier to determine the pressure of water, the ogous to saying that the atmospheric pressure at energy content of water is usually categorized on sea level is equal to I atmosphere, is equivalent to the basis of pressure. a column of water of about 33 feet, or to a column The hull of a submarine must be sturdily built to of mercury of about 76 centimeters. These are ² withstand the great pressure encountered in a also equal to a pressure of 14.7 lb/in. dive far below the ocean's surface. The column At the 10-centimeter depth, the water pressure (head) of water above the submarine exerts pres- in the beaker is one half of that at the 20- cen- ² sure on the hull of the submarine. If the water timeter depth and, therefore, is 10 g/cm . The pressure exerted against the submarine was di- water pressure decreases with distance toward rected against the blades of a turbine, the energy the surface and becomes 0 g/cm², at the free 58 SOIL WATER

open water surface and moving upward into a capillary tube, water pressure continues to decrease. Thus, the water pressure in the capillary tube is less than zero, or is negative. As shown in Figure 5.5, the water pressure decreases in the capillary tube with increasing height above the open water surface. At a height of 20 centimeters above the water surface in the beaker, the water pressure in the capillary is equal to -20 g/cm . Figure 5.5 also shows that at this same height above the water surface, the water pressure in unsaturated soil is the same at the 20-centimeter height. The two pressures are the same at any one height, and there is no net flow of water from the soil or capillary tube after an equilibrium condi- FIGURE 5.4 A beaker with a cross-sectional area of tion is established. 100 square centimeters contains 2,000 grams of water Applying the consideration of water in a capil- when the water is 20 centimeters deep. At the water lary tube to an unsaturated soil, allows the follow- surface, water pressure is 0; the water pressure ing statements: increases with depth to 20 grams per square centimeter at the bottom.

FIGURE 5.5 water surface, as shown in Figure 5.4. In water- Water pressure in a capillary tube decreases with increasing distance above the water saturated soil, the water pressure is zero at the in the beaker. It is -20 grams per square centimeter surface of the water table and the water pressure at a height 20 centimeters above the water surface. i ncreases with increasing soil depth. Since the water column of the capillary tube is continuous through the beaker and up into the soil column, the water pressure in the soil 20 centimeters above the water level in the beaker is also -20 grams per square centimeter. Pee Relaionhip in Unaaed Soil If the tip of a small diameter glass or capillary tube is inserted into the water in a beaker, adhesion causes water molecules to migrate up the interior wall of the capillary tube. The cohesive forces between water molecules cause other water molecules to be drawn up to a level above the water in the beaker. Now, we need to ask: "What is the pressure of the water in the capillary tube?" and "How can this knowledge be applied to unsaturated soils?" As shown previously, the water pressure decreases from the bottom to the top of a beaker filled with water and, at the top of the water surface, the water pressure is zero. Beginning at the THE SOIL WATER POTENTIAL 59

1. Water in unsaturated soil has a negative pres- reference pool (which is at a lower elevation) to sure or is under tension. soil that is at a higher elevation, the sign is posi- 2. The water pressure in unsaturated soil de- tive. The higher the water above the reference creases with increasing distance above a free point, the greater the gravitational water potential. water surface or water table. Visualize a water-saturated soil that is underlain at the 1-meter depth with plastic tubing containing small holes. Normally, the plastic tubing allows gravitational water to drain out of the soil. THE SOIL WATER POTENTIAL Suppose, however, that the tube outlet is closed Most of the concerns about soil water involve and the soil remains water saturated. The gravita- movement: water movement into the soil surface, tional water potential at the top of the soil that is water movement from the soil surface downward saturated, relative to the plastic tubing, is equal to through soil, and movement of water from the soil a 1-meter water column. This is the same as say- to, and into, roots, microorganisms, and seeds. ing that the water pressure is equal to a 1-meter The pressure of soil water, which is an indication column of water in reference to the point of water of tendency for soil water to move, is expressed by drainage through the tubing. the oil ae poenial. Technically, the soil wa- If the drainage tube outlet is opened, gravita- ter potential is defined as the amount of work that tional water will leave the water-saturated soil and must be done per unit quantity of water to trans- drain away through the plastic tubing. The soil port or move reversibly (without energy loss due will desaturate from the surface downward. The to friction) and isothermally (without energy loss water pressure or gravitational potential at the top due to temperature change) an infinitesimal of water-saturated soil will decrease as the water quantity of water from a pool of pure water at level drops. When the gravitational water has specified elevation and at atmospheric pressure been removed, and drainage stops, the top of the to the soil water at the point under consideration. new level of water-saturated soil will be at the It is important to note that the water potential is same elevation as the drainage tubing (the refer- the amon oph ok needed o moe ae phom a ence point). At this elevation, the gravitational epheence pool o anohe poin. potential will be zero. The symbol for the water potential is psi, 0. The total water potential, q t , is made up of several subpotentials, including the gravitational, matric, The Maic Poenial and osmotic. When a soil is unsaturated and contains no gravitational water, the major movement of water is laterally from soil to plant roots. The important The Gaiaional Poenial forces affecting water movement are adhesion The gravitational potential, g , is due to the and cohesion. Adhesion and cohesion effects are position of water in a gravitational field. The gravi- intimately affected by the size and nature of pri- tational potential is very important in water- mary soil particles and peds. The resulting physi- saturated soils. It accounts primarily for the move- cal arrangement of surfaces and spaces, owing to ment of water through saturated soils or the texture and structure, is the soil matrix. The inter- movement of water from high to low elevations. action of the soil matrix with the water produces Customarily, the soil water being considered is the matric water potential. The matric potential higher in elevation than the reference pool of pure is 1m . water. Since the gravitational potential represents When rain falls on dry soil, it is analogous to the work that must be done to move water from a moving water molecules from a pool of pure water 60 SOIL WATER to the water films on soil particles. No energy osmotic potential has little, if any, effect on water input is required for the movement of water in movement within the soil, since diffusion of the raindrops to water films around dry soil particles. hydrated ions creates a uniform distribution and In fact, a release of energy occurs, resulting in a the hydrated ions do not contribute to a differen- decrease in the potential of the water. Adsorbed tial in matric potential in one region of the soil as water has lower potential to do work than water in compared to another. raindrops. In other words, negative work is required to transfer water from raindrops to the films on soil particles. This causes the matric Meaemen and Epeion of potential to have a negative sign. Wae Poenial The matric potential varies with the content of Since the gravitational potential is the distance soil water. The drier a soil is, the greater is the between the reference point and the surface of the tendency of the soil to wet and the greater is the water-saturated soil, or the water table, it can be release of energy when it becomes wetted. In fact, measured with a ruler. Reference has been made when oven dry clay is wetted, a measureable in- to a gravitational potential equal to a meter-long crease in temperature occurs due to the release of water column. Water potentials are frequently ex- energy as heat. This release of heat is called the pressed as bars that are approximately equal to heat oph wetting. Generally, the drier a soil is, the atmospheres. A bar is equivalent to a 1,020 cen- lower is the matric potential. The number timeter column of water. preceded by a negative sign becomes larger (be- The matric potential can be measured in sev- comes more negative) when a soil dries from field eral ways. Vacuum gauge potentiometers consist capacity to wilt point. In reference to the previous of a rigid plastic tube having a porous fired clay situation where the gravitational water had just cup on one end and a vacuum gauge on the other. exited a soil with drainage tubing at a depth of 1 The potentiometer is filled with pure water, and at meter, the matric water potential in unsaturated this point the vacuum gauge reads zero. The soil at the soil surface is equal to a -1-meter potentiometer is then buried in the soil so that the column of water. porous cup has good contact with the surrounding soil (see Figure 5.6). Since the potential of pure water in the potentiometer is greater than The Omoic Poenial water in unsaturated soil, water will move from

The osmotic potential, o, is caused by the forces the potentiometer into the soil. At equilibrium, the involved in the adsorption of water molecules by vacuum gauge will record the matric potential. ions (hydration) from the dissolution of soluble Vacuum gauge potentiometers work in the range salt. Since the adsorption of water molecules by 0 to -0.8 bars, which is a biologically important ions releases heat (energy of hydration), the sign range for plant growth. They are used in irrigated of the osmotic potential is also negative. agriculture to determine when to irrigate. The osmotic potential is a measure of the work A pressure chamber is used to measure matric that is required to pull water molecules away from potentials less than -0.8 bars. Wet soil is placed hydrated ions. Normally, the salt content of the on a porous ceramic plate with very fine pores, soil solution is low and the osmotic potential has which is confined in a pressure chamber. Air pres- little significance. In soils containing a large sure is applied, and the air forces water through amount of soluble salt (saline soils), however, the the fine pores in the ceramic plate. At equilibrium, osmotic potential has the effect of reducing water when water is no longer leaving the soil, the air uptake by roots, seeds, and microorganisms. The pressure applied is equated to the matric poten- SOIL WATER MOVEMENT 61

FIGURE 5.6 Soil water potentiometers for measuring the matric potential. The one on the left has been removed from the soil to show the porous clay cup.

tial. At equilibrium, an air pressure of 15 bars is of water flow is directly related to the water poten- equal to a matric potential of -15 bars. This tech- tial difference and inversely related to the flow nique is used to determine the matric potential in distance. the range of -0.3 to -15 bars, which is the range The velocity of water flow is also affected by the between field capacity and the permanent wilting soil's ability to transmit water or the hydraulic point, respectively. conductivity (k). Water flow through large pores is Many books and publications express the water faster than through small pores; flow is faster potential in bar units. However, kilopascals and when the conductivity is greater. Therefore, the megapascals are being used more frequently to velocity of flow, V, is equal to the water potential express the water potential. A bar is equivalent to gradient times the hydraulic conductivity, k. Math- 100 kilopascals (kPa). Using kilopascals, the wa- ematically, ter potential at field capacity is -30 kPa and at the V = kph permanent wilting point is -1,500 kPa. The hydraulic conductivity, or the ability of the soil to transmit water, is determined by the nature and size of the spaces and pores through which SOIL WATER MOVEMENT the water moves. The conductivity of a soil is Water movement in soils, and from soils to plant analogous to the size of the door to a room. As the roots, like water cascading over a dam, is from door to a room becomes larger, the velocity o1 regions of higher-energy water to regions of lower- speed at which people can go in or out of the energy water. Thus, water runs downhill. room increases. As long as the door size remain: The driving force for water movement is the constant, the speed with which people can move difference in water potentials between two points. into or out of the room remains constant. As long The water potential gradient (t) is the water as the physical properties of a soil (including potential difference between two points divided water content) remain constant, k is constant. by the distance between the two points. The rate Conductivity (k) is closely related to pore size 62 SOIL WATER

Water flow in a pipe is directly related to the fourth This equation is named after Darcy, who stud- power of the radius. Water can flow through a ied the flow of water through a porous medium large pore, having a radius 10 times that of a small with a setup like that shown in Figure 5.7. The pore, 10,000 times faster than through the small setup includes a column in which a 12- pore. In water-saturated soils, water moves much centimeter-thick layer of water rests on top of a more rapidly through sands than clays because 15-centimeter-thick layer of water-saturated soil. sands have larger pores. The larger pores give the In this case, the potential difference between the sandy soils a greater ability to transmit water or a two points is equal to 12 centimeters (point A to greater conductivity when saturated as compared point B) plus 15 centimeters (point B to point C) to clayey soils. or 27 centimeters; h equals 27 centimeters. The The size of pores through which water moves distance of flow through soil, d, is 15 centimeters. through soil, however, is related to the water con- If the amount of water that flowed through the soil tent of the soil. In saturated soil, all soil pores are in 1 hour was 360 cm ³ , and the cross-sectional water filled and water moves rapidly through the area of the soil column is 100 cm² , k can be largest macropores. At the same time, water calculated as follows: movement in the smaller pores is slow or nonexis- 27 cm tent. As the water content of soil decreases, water 360 cm ³ = k x x 100 cm² x I hr moves through pores with decreasing size, be- 15 cm cause the largest pores are emptied first, followed Then, k = 2 cm/hr by the emptying of pores of decreasing size. The value fork of 2 cm/hr is in the range of soils Therefore, k (hydraulic conductivity of the soil) is with moderate saturated hydraulic conductivity; closely related to soil water content; k decreases i n the range of 0.5 cm/hr to 12.5 cm/hr. Soils with with decreasing soil water content. slow saturated hydraulic conductivity range from less than 0.125 cm/hr to 0.5 cm/hr. Soils with rapid saturated hydraulic conductivity have a con- Wae Moemen in Saaed Soil ductivity in the range 12.5 cm/hr to more than As shown previously, the potential gradient (ph) is the difference in water potentials between the two FIGURE 5.7 A setup similar to that which Darcy points of flow divided by the distance between the used to study water movement in porous materials. points of flow. The difference in water potentials between two points in water-saturated soil is the head (h). The gradient, f, is inversely related to the distance of flow through the soil, d. Therefore, SOIL WATER MOVEMENT 63

25 cm/hr. Thus, a sand soil could have a saturated loss of water by evaporation. As a consequence, hydraulic conductivity that is several hundred gravity plays a minimum role in the movement of times greater than that of a clay soil. water in water-unsaturated soil. According to Darcy's equation, if the distance Darcy's law also applies to the movement of of water movement through soil is doubled, the water in unsaturated soil. The value for h, the quantity of water flow is halved. This is an impor- hydraulic head, becomes the difference in the tant consideration in soil drainage, because the matric potentials between the points of flow. For further soil is from the drain lines, the longer it water movement from soil to roots, h will com- will take for water to exit the soil. Therefore, the monly be a few hundred kPa (a few bars); distance between drainage lines is dependent on for example, the difference between -100 and the hydraulic conductivity. Another important de- -300 kPa (-1 to -3 bars). cision based on the saturated hydraulic conduc- A most important consideration for unsaturated tivity includes determining the size of seepage flow is the fact that unsaturated hydraulic conduc- beds for septic tank effluent disposal. tivity can vary by a factor of about 1 million within the range that is biologically important. Unsatu- rated hydraulic conductivity decreases rapidly Wae Moemen in Unaaed Soil when plants absorb water and soils become drier If water is allowed to drain from a saturated soil, than field capacity. For the Geary soil (see Table water leaves the soil first and most rapidly via the 5.1) , k near field capacity at -33 kPa is 1.1 x 10 - ' largest pores. As drainage proceeds over time, compared with 6.4 x 10 -5 cm/day for a potential water movement shifts to smaller and smaller of -768 kPa. This means that the conductivity in pores. The result is a rapid decrease in both hy- the soil at field capacity is more than 1,700 times draulic conductivity and in the rate of drainage, or greater than in soil midway between field capacity movement of water from the soil. Field soils with and the permanent wilting point. The water in an average saturated hydraulic conductivity will, unsaturated soil is very immobile and significant essentially, stop draining within a day or two after movement occurs only over very short distances. a thorough wetting, meaning the gravitational wa- The distance between roots must be small to re- ter has exited the soil and the soil is retaining the cover this water. It is not surprising that Dittmer maximum amount of water against the force of (1937) found that a rye plant, growing in a cubic gravity. Such soil in the field is at field capacity foot of soil, increased root length about 3 miles and will have a matric potential varying from per day when consideration was made for root about -10 to -30 kPa (-0.1 to -0.3 bars). The hair growth. When plants wilt for a lack of water, data in Table 5.1 show that the conductivity of the the wilting is due more to the slow transmission Geary silt loam decreased from 9.5 cm/day at rate of water from soil to roots rather than a low water-saturation (matric potential 0) to only water content of the soil per se. 1.1 x 10 - ' near field capacity (matric potential equal to -33 kPa or -0.3 bars). Thus, the hydraulic conductivity at field capacity was about 1 per- Wae Moemen in Saified Soil cent as great as at saturation. Stratified soils have layers, or horizons, with dif- Gravity is always operating. After field capacity ferent physical properties, resulting in differences is attained, however, matric forces control water in hydraulic conductivity. As a consequence, the movement. Additional movement is very slow and downward movement of water may be altered over short distances. The flow tends to be hori- when the downward moving water front encoun- zontally from soil to roots, except for some verti- ters a layer with a different texture. cal movement at the soil surface, resulting from When rain or irrigation water is added to the 64 SOIL WATER

TABLE 5.1 Volume Water Content, Hydraulic Conductivities, and Matric Potentials of a Loam and Silt Loam Soil

Data from Hanks, 1965. surface of dry, unsaturated soils, the immediate If the water is moving through loam soil and the soil surface becomes water saturated. Infiltration, wetting front encounters a dry sand layer, the wet- or water movement through the soil surface, is ting front stops its downward movement. The wet- generally limited by the small size of the pores in ting front then continues to move laterally above the soil surface. Water does not rush into the the sand layer, as shown in Figure 5.8, because underlying drier unsaturated soil, but moves the unsaturated hydraulic conductivity in the dry slowly as unsaturated flow. The downward flow of sand is less than that of the wet silt loam soil water below the soil surface is in the micropores, above. Water in the wetting front is moving as a while the macropores remain air-filled. The water liquid and, in dry sand, there are essentially no moves slowly through the soil as a wetting front, continuous water films between sand grains. That creating a rather sharp boundary between moist is, the path of water movement is interrupted be- and dry soil. Similarly, water moves slowly from cause the water films in many adjacent sand an irrigation furrow in all directions. Matric forces grains do not touch each other. The abrupt stop- (cohesion and adhesion) control water move- page of the water front, shown in Figure 5.8, is ment; water tends to move upward almost as rap- analagous to a vehicle moving at a high speed and idly as downward. abruptly stopping when it reaches a bridge that SOIL WATER MOVEMENT 65

FIGURE 5.8 Photographs illustrating water movement in stratified soil where water is moving as unsaturated flow through a silt loam into a sand layer. In the upper photograph, water movement into the sand is inhibited by the low unsaturated hydraulic conductivity of dry sand. The lower photograph shows that after an elapsed time of 1.5 to 5 hours, the water content in the lower part of the silt loam soil became high enough to cause water drainage downward into the sand. (Photographs courtesy Dr. W. H. Gardner, Washington State University.)

has collapsed. There is a lack of road continuity any sand and gravel layers underlying loamy soils, where the bridge has collapsed. The distance be- enables the loam soil layer to retain more water tween particles in clay soils is small, and there is than if the the soil had a loam texture throughout. greater likelihood of water films bridging between A gravel layer under golf greens results in greater the particles in dry clay soil as compared to dry water storage in the overlying soil. Conversely, sand. This gives dry clay soils greater hydraulic during periods of excessive rainfall, drainage will conductivity than dry sands. occur and prevent complete water saturation of As the soil under the irrigation furrow, and the soil on the green. above the sand layer, becomes wetter over time, If a clay layer underlies a loamy soil, the wetting the water potential gradient between the moist front will immediately enter the clay. Now, soil and dry soil increases. In time, the water however, the clay's capacity to transmit water is breaks through the silt loam soil and enters the very low by comparison, and water will continue sand layer, as shown in the lower part of Figure to move downwward in the loam soil at a faster 5.8. The low unsaturated hydraulic conductivity of rate than through the clay layer. Under these con- 66 SOIL WATER ditions, the soil saturates above the clay layer, as PLANT AND SOIL WATER RELATIONS it does when a claypan exists in a soil. From this Plants use large quantities of water, and this sec- discussion it should be apparent that well-drained tion considers the ability of plants to satisfy their soils tend never to saturate unless some underly- water needs by absorbing water from the soil and ing layer interrupts the downward movement of a the effect of the soil water potential on nutrient wetting front and soil saturates from the bottom uptake and plant growth. upward. At the junction of soil layers or horizons of different texture, downward moving water tends Aailable Wae Sppling Poe to build-up and accumulate. This causes a tempo- of Soil rary increase in the availability of water and of The water-supplying power of soils is related to nutrients, unless very wet and saturated soil is the amount of available water a soil can hold. The produced, and results in increased root growth available water is the difference in the amount of and uptake of nutrients and water near or at these water at field capacity (-30 kPa or -0.3 bar) and junctions. the amount of water at the permanent wilting point (-1,500 kPa or -15 bars). The amount of available water is related to both Wae Vapo Moemen texture and structure, because it is dependent on When soils are unsaturated, some of the pore the nature of the surfaces and pores, or soil ma- space is air filled and water moves through the trix. Soils high in silt (silt loams) tend to have the pore space as vapor. The driving force is the dif- most optimum combination of surfaces and ference in water potentials as expressed by differ- pores. They have the largest available water- ences in vapor pressure. Generally, vapor pres- holding capacity, as shown in Figure 5.9. These sure is high in warm and moist soil and low in soils contain about 16 centimeters of plant- cold and dry soil. Vapor flow, then, occurs from available water per 100 centimeters of depth, or 16 warm and moist soil to and into cold and dry percent by volume. This is about 2 inches of water soil. In the summer, vapor flow travels from the per foot of soil depth. When such soil is at the warmer upper soil horizons to the deeper and permanent wilting point, a 2-inch rain or irri- colder soil horizons. During the fall, with transi- gation, which infiltrates completely, will bring the tion to winter, vapor flow tends to be upward. It soil to field capacity to a depth of about 1 foot. is encouraged by a low air temperature and up- From Figure 5.9, it can be noted that sands have ward water vapor movement, and condensation the smallest available water-holding capacity. onto the surface of the soil commonly creates This tends to make sands droughty soils. Another a slippery smear in the late night and early morn- reason why sandy soils tend to be droughty is that ing hours. most of the available water is held at relatively Soil conductivity for vapor flow is little affected high water potential, compared with loams and by pore size, but it increases with an increase in clays. It should be noted in Figure 5.10 that field both total porosity and pore space continunity. capacity for sand is more likely nearer -10 kPa Water vapor movement is minor in most soils, but (-0.1 bar) than -30 kPa (-0.3 bar) as in loamy it becomes important when soils crack at the sur- and clayey soils. This means that much of the face in dry weather and water is lost from deep plant available water in sandy soils can move within the soil along the surfaces of the cracks. rapidly to roots when water near roots is depleted, Tillage that closes the cracks, or mulches that because of the relatively high hydraulic conduc- cover the cracks, will reduce water loss by evapo- tivity at these relatively high potentials. Plants ration. growing on clay soils, by contrast, may not absorb PLANT AND SOIL WATER RELATIONS 67

FIGURE 5.9 Relationship of soil texture to available water-holding capacity of soils. The difference between the water content at field capacity and the water content at the permanent wilting point is the available water content. as much water as they could use during the day sands tend to be more droughty than clays be- because of lower hydraulic conductivity and cause they retain less water at field capacity, and slower replacement of water near roots when wa- the water retained is consumed more rapidly. ter is depleted. This results in a tendency to conserve the available water in fine-textured soils. In essence, the water retained at field capacity in Wae Upake fom Soil b Roo sands is more plant available than the water in Plants play a rather passive role in their use of fine-textured soils at field capacity. Therefore, water. Water loss from leaves by transpiration is

FIGURE 5.10 Soil water characteristic curves for three soils. Field capacity of the sandy loam is about -0.1 kPa x 10 -2 (-10 kPa) and for the finer-textured soils is about -0.3 kPa x 10 -2 (-30 kPa).

0 68 SOIL WATER

mainly dependent on the surrounding environ- Dinal Paen of Wae Upake ment. Water in the atmosphere has a much lower Suppose that it is a hot summer morning, and water potential than water in a leaf, causing the there has been a soaking rain or irrigation during atmosphere to be a sink for the loss of water from the night. The soil is about at field capacity and the plant via transpiration. The water-conducting water availability is high. The demand for water is tissue of the leaves (xylem) is connected to the nil, however, because the water-potential differ- xylem of the stem, and a water potential gradient ences in the leaves, roots, and soil are small. develops between leaves and stems due to water There is no significant water gradient and no wa- loss by transpiration. The xylem of the stem is ter uptake. After the sun rises and the temperature connected to the xylem of the roots, and a water increases, thus decreasing relative humidity, the potential gradient is established between the stem water potential gradient increases between the and roots. The water potential gradient estab- atmosphere and the leaves. The leaves begin to lished between roots of transpiring plants and soil transpire. A water potential gradient is established causes water to move from the soil into roots. This between the leaves, roots, and soil. Roots begin water potential gradient, or continuum, is called water uptake. The low water conductivity in (the the soil-plant-atmosphere-continuum, or SPAC. plant causes the water potential difference be- Some realistic water potentials in the continuum tween leaves and roots, and between roots and during midday when plants are actively absorb- soil, to increase to a maximum at about 2 P.M., as ing water are -50,000 kPa (-500 bars) in the shown in Figure 5.11. This is a period of greater atmosphere, - 2,500 kPa (-25 bars) in the leaf, water loss than water uptake, which creates a - 800 kPa (-8 bars) in the root, and -700 kPa water deficit in the leaves. It is normal for these (-7 bars) in the soil. There is considerable resis- leaves to be less than fully turgid and appear tance to upward water movement in the xylem, so slightly wilted. that a considerable water potential gradient can After 2 P.M., decreasing temperature and in- be produced between the leaf and the root. creasing relative humidity tend to reduce water

FIGURE 5.11 Diurnal changes in water potential of soil, roots, and leaves during a period of soil drying. (After Slatyer, 1967.) PLANT AND SOIL WATER RELATIONS 69 loss from leaves. With continued water uptake, -6,000 kPa (-10 bars to -60 bars), which repre- the plant absorbs water faster it is lost, and the sents a rather narrow range in differences in soil water potential gradient between leaves and soil water content. Desert plants can thrive with very decreases. The water deficit in the plant is de- low water potentials and, typically, have some creased, and any symptoms of wilting disappear. special feature that enables them to withstand a By the next morning, the water potential gradient limited supply of soil water. Some plants shed between the leaves and soil is eliminated (see their leaves during periods of extreme water Figure 5.11). stress. As this pattern is repeated, the soil dries and the hydraulic conductivity decreases rapidly and the distance water must move to roots increases. In Paen of Wae Remoal fom Soil essence, the availability of soil water decreases With each passing summer day, water moves over time. As a consequence, each day the po- more slowly from soil to roots as soils dry, and the tential for water uptake decreases and the availability of the remaining water decreases. This plant increasingly develops more internal water encourages root extension into soil devoid of deficit, or water stress, during the afternoon. roots where the water potential is higher and wa- The accumulated effects of these diurnal ter uptake is more rapid. For young plants with a changes, in the absence of rain or irrigation small root system, there is a continual increase in or the elongation of roots into underlying moist rooting depth to contact additional supplies of soil, are shown in Figure 5.11. Eventually, available water in the absence of rain or irrigation leaves may wilt in midday and not recover their during the summer. In this way the root zone is turgidity at night. Permanent wilting occurs, the progressively depleted of water as shown in Fig- soil is at the permanent wilting point, and the ure 5.12. When the upper soil layers are rewetted soil has a water potential of about -1,500 kPa by rain or irrigation, water absorption shifts back (-15 bars). toward the surface soil layer near the base of the The sequence of events just described is typical plant. This pattern of water removal results in: where roots are distributed throughout the soil (1) more deeply penetrating roots in dry years from which water uptake occurs. For the se- than in wet years, and (2) greater absorption of quence of events early in the growing season of an water from the uppermost soil layers, as com- annual plant, continual extension of roots into pared to the subsoil layers, during the growing underlying moist soil may occur to delay the time season. Crops that have a long growing season when the plant wilts in the absence of rain or and a deep root system, like alfalfa, absorb a irrigation. greater proportion of their water below 30 cen- The permanent wilting point is not actually a timeters (see Figure 5.13). point but a range. Experiments have shown that some common plants could recover from wilting when the potential decreased below -1,500 kPa Soil Wae Poenial Ve (-15 bars). High temperature and strong winds, Plan Goh on the other hand, can bring about permanent Most plants cannot tolerate the low oxygen levels wilting at potentials greater than - 1,500 kPa. If a of water-saturated soils. In fact, water saturation plant can survive and, perhaps, make slow growth of soil kills many kinds of plants. These plants with a small water demand, the soil water po- experience an increase in plant growth from satu- tential may be much less than -1,500 kPa. ration to near field capacity. As soils dry beyond For most soils and plant situations, the perma- field capacity, increased temporary wilting during nent wilting range appears to be about -1,000 to the daytime causes a reduction in photosynthesis. 70 SOIL WATER

FIGURE 5.12 Pattern of water used from soil by sugar beets during a 4-week period following irrigation. Water in the upper soil layers was used first. Then, water was removed from increasing soil depths with time since irrigation. (Data from Taylor, 1957.)

FIGURE 5.13 Percentage of water used from each Thus, plant growth tends to be at a maximum 30-centimeter layer of soil for crops produced with when growing in soil near field capacity, where irrigation in Arizona. Onions are a shallow-rooted the integrated supply of both oxygen and water annual crop that is grown in winter and used a total of 44 centimeters of water. Alflalfa, by contrast, grew are the most favorable. the entire year and used a total of 186 centimeters of During periods of water stress, the damage to water, much of it from deep in the soil. (Data from plants is related to the stage of development. The Arizona Agr. Exp. Sta. Tech. Bull. 169, 1965.) data in Table 5.2 show that four days of visible wilting during the early vegetative stage caused a 5 to 10 percent reduction in corn yield compared with a 40 to 50 percent reduction during the time of silk emergence and pollination. Injury from water stress at the dough stage was 20 to 30 percent.

TABLE 5.2 Effect of Four Days of Visible Wilting on Corn Yield SOIL WATER REGIME 71

Role of Wae Upake fo Nien Upake Mass flow carries ions to the root surfaces, where they are in a position to be absorbed by roots. An increased flow of water increases the movement of nutrient ions to roots, where they are available for uptake. This results in a positive interaction between water uptake and nutrient uptake. Droughts reduce both water and nutrient uptake. Fertilizers increase drought resistance of plants when their use results in greater root growth and rooting depth (see Figure 5.14).

FIGURE 5.14 Fertilizer greatly increased the growth of both tops and roots of wheat. Greater root density and rooting depth resulted in greater use of stored soil water. (Photograph courtesy Dr. J. B. Fehrenbacher, University of Illinois.)

FIGURE 5.15 Climatic data and average, soil water storage capacity were used to construct the water regime for soils near Memphis, Tennessee. (Data from Thornthwaite and Mather, 1955.)

SOIL WATER REGIME

The oil ae egime expresses the gradual changes in the water content of the soil. For example, a soil in a humid region may at some time during the year have surplus water for leaching, soil water depletion by plant uptake, and water recharge of dry soil. These conditions are illustrated in Figure 5.15 for a soil with an average water-storage capacity in a humid region, such as near Memphis, Tennessee. The early months of the year (January to May) are characterized by surplus water (see Figure 5.15). The soil has been recharged with water by 72 SOIL WATER

rains from the previous fall, when the soil was differences and inversely related to the distance of brought to field capacity. The precipitation in the flow. early part of the year is much greater than the The rate of water flow is also directly related to potential for water evaporation (potential evapo- soil hydraulic conductivity. The hydraulic con- transpiration), and this produces surplus water. ductivity decreases rapidly as soils dry. This water (gravitational) cannot be held in a soil Plant uptake of water reduces the water content at field capacity, and it moves through the soil, of the soil and greatly decreases the hydraulic producing leaching. During the summer months conductivity. Thus, as soil dries, water movement the potential evapotranspiration is much greater to roots becomes slower. Plants wilt when water than the precipitation. At this time, plants deplete movement to roots is too slow to satisfy the plant's stored soil water. Since there is insufficient stored demand for water. water to meet the potential demand for water During the course of the year the major changes (potential evapotranspiration), a water deficit is in soil water content include: recharge of dry soil, produced. This means that most of the plants surplus water and leaching, and depletion of could have used more water than was available. stored water. Water deficits occur when water Abundant rainfall in some years prevents a deficit; loss from the soil by evapotranspiration exceeds in very dry years, the water deficit may be quite the precipitation plus stored water for a period of large and a drought may occur. In the fall the time. potential evapotranspiration drops below the level of the precipitation, and water recharge of dry soil occurs. Recharge, in this case, is com- REFERENCES pleted in the fall, and additional pecipitation contributes mainly to surplus (gravitational) water Classen, M. M. and R. H. Shaw. 1970. "Water Deficit and leaching. Leaching occurs only during the Effects on Corn:II Grain Components." Agron. J. period when surplus water is produced. 62:652-655. Dittmer, H. J. 1937. "A Quantative Study of the Roots The water regime in desert climates is charac- and Root Hairs of a Winter Rye Plant." Am. J. Bot. terized by little water storage, a very small soil 24:417-420. water depletion, a very large deficit, and no sur- Erie, L. J., 0. F. French, and K. Harris. 1965. "Con- plus water for leaching. sumptive Use of Water by Crops in Arizona." Aiona Agr. Exp. Sta. Tech. Bull. 169. SUMMARY Gardner, W. H. 1968. "How Water Moves in the Soil." Crops and Soils. November, pp. 7-12. The water in soils has an energy level that varies Hanks, R. J. 1965. "Estimating Infiltration from Soil with the soil water content. The energy level of Moisture Properties." J Soil Water Conserv. 20: soil water increases as the water content in- 49-51. creases and decreases as the water content of the Hanks, R. J. and G. L. Ashcroft. 1980. Applied Soil Physics. Springer-Verlag, New York. soil decreases. This produces a soil water-energy Slayter, R. 0. 1967. Plant-water Relationships. Aca- continuum. demic Press, New York. The energy level of soil water is expressed as Taylor, S. A. 1957. "Use of Moisture by Plants." Soil, the soil water potential, commonly as bars or USDA Yearbook. Washington, D.C. kilopascals. Thornthwaite, C. W. and J. R. Mather. 1955. "Clima- Water moves within soil and from soil to roots tology and Irrigation Scheduling." Weekly Weather in response to water potential differences. Water and Crop Bulletin. National Summary of June 27, movement is directly related to water potential 1955. CHAPTER 6

SOIL WATER MANAGEMENT

The story of water, in a very real sense, is the story subhumid climates. Techniques for water conser- of humankind. Civilization and cities emerged vation are designed to increase the amount of along the rivers of the Near East. The world's water that enters the soil and to make efficient use oldest known dam in Egypt is more than 5,000 of this water. years old. It was used to store water for drinking and irrigation, and perhaps, to control flood- Modifying the Infiltration Rate waters. As the world's population and the need for Infiltration is the downward entry of water through food and fiber increased, water management became more important. Now, in some areas of the soil surface. The size and nature of the soil rapidly increasing urban development and limited pores and the water potential gradient near the soil surface are important in determining the water supply, competition for the use of water has arisen between agricultural and urban users. amount of precipitation that infiltrates and the amount that runs off. High infiltration rates, there- There are three basic approaches to water man- fore, not only increase the amount of water stored agement on agricultural land: (1) conservation for plant use but also reduce flooding and soil of natural precipitation in subhumid and arid erosion. regions, (2) removal of water from wetlands, and Surface soils protected by vegetation in a natu- (3) addition of water to supplement the amount of ral forest or grassland tend to have a higher in- natural precipitation (irrigation). Two important filtration rate during rains than exposed soils in needs for water management in urban areas in- cultivated fields or at construction sites. In an clude land disposal of sewage effluent and prescription athletic turf. experiment in which excessive water was applied, the protection of the soil surface by straw or burlap maintained high and constant infiltration WATER CONSERVATION rates, as shown in Figure 6.1. When the straw and Water conservation is important in areas in which burlap were removed, the infiltration rates de- large water deficits occur in soils within arid and creased rapidly. The impact of raindrops on ex-

73 74 SOIL WATER MANAGEMENT

faster than the same volume of smaller pores. In natural forest and grassland areas, an abundant food supply at the soil surface encourages earthworms, resulting in large surface soil burrows that are protected from raindrop impact by the vegetative cover. In fields, it is common for infiltration rates to decrease over time during rainstorms. This is brought about by a combination of factors. First, a FIGURE 6.1 Effect of straw and burlap and their reduction in hydraulic conductivity occurs at the removal on soil infiltration rate. (Source: Miller and soil surface, owing to physical changes. Second, Giffore, 1974.) as the soil becomes wet, and the distance of water movement increases to wet the underlying dry soil, the velocity of water flow decreases. In- posed soil breaks up soil peds and forms a crust. filtration rates from irrigation furrows, however, a The average pore size in the surface soil de- few hours after the start of irrigation have been creases, causing a reduction in the infitration rate. caused by earthworms that moved toward the Infiltration is also decreased by overgrazing and moist soil and entered the waterfilled furrows, deforestation, or by any practices that remove leaving behind channels that increased infil- plant cover and/or cause soil compaction. The tration. bare field in Figure 6.2 shows the detrimental Many farmers modify the soil surface condi- effects of raindrop impact on soil structure at the tions with contour tillage and terraces. These soil surface, decreased water infiltration and, practices temporarily pond the water and slow then, increased water runoff and soil erosion. down the rate of runoff, which results in a longer Since water movement into and through a soil time for the water to infiltrate the soil. In some pore increases exponentially as the radius in- situations, where there is little precipitation, run- creases, a single earthworm channel that is open off water is collected for filling reservoirs. Then, at the soil surface may transmit water 100 times surface soils with zero infiltration rates are desir-

FIGURE 6.2 Effects of raindrop impact on soil structure at the soil surface and on water infiltration and water runoff. The soil is unprotected by vegetation or crop residues, resulting in a greatly reduced infiltration rate and greatly increased runoff and soil erosion. Such effects accentuate the differences in the amount of water stored in soils for plant growth on the ridge tops, side slopes, and in the low areas where runoff water accumulates and sedimentation of eroded soil occurs. WATER CONSERVATION 7 5 able. In these cases, the important product pro- yellow-colored strips where wheat was produced, duced by the soil is runoff water. as shown in Figure 6.3. In order to explain how water storage occurs in fallowed land, both evaporation of water from the soil surface and water Summer Fallowing movement within the soil must be considered. Most of the world's wheat production occurs in After a wheat crop has been harvested, the wa- regions having a subhumid or semiarid climate. ter storage period begins. The surface soil layer is In the most humid parts of these regions, soils air dry and the underlying soil is at the approxi- have enough stored water (water from precipi- mate permanent wilting point (see Figure 6.4). tation in excess of evaporation during the winter Rain on dry soil immediately saturates and in- and early spring), plus that from precipitation dur- filtrates the soil surface, and water moves down- ing the growing season, to produce a good crop of ward as a wetting front, similar to water moving wheat in almost every year. In the drier part of out of an irrigation furrow as shown in Figure 5.8. these regions, the water deficit is larger and in The depth of water penetration depends on the most years insufficient water is available to pro- amount of infiltration and the soil texture. Each duce a profitable crop. Here, a practice called centimeter of water that infiltrates will moisten summer phallowing is used. Summer fallowing about 6 to 8 centimeters of loamy soil, which is means that no crop is grown and all vegetative near the permanent wilting point. A heavy rain- growth is prevented by shallow tillage or herbi- storm will produce an upper soil layer at field cides for a period of time, in order to store water capacity, whereas the underlying soil remains at for use by the next crop. the wilting point (see Figure 6.4). After wheat is harvested, the land is left un- After the rain, some water will evaporate from planted. No crop is grown and weeds are con- the soil surface. This creates a water potential trolled by shallow cultivation or herbicides to gradient between the surface of the drying soil minimize the loss of water by transpiration. Dur- and underlying moist soil, and some water in the ing the year of the fallow period, there is greater underlying moist soil will migrate upward. This water recharge than if plants were allowed to grow phase of soil drying is characterized by rapid wa- and transpire. This additional stored water, to- ter loss. As drying near the soil surface occurs, a gether with the next year's precipitation, is used rapid decrease in hydraulic conductivity occurs at by the wheat. Fallowing produces a landscape of the soil surface. Water migration upward is greatly alternating dark-colored fallow strips of soil and reduced, and the evaporative demand at the sur-

FIGURE 6.3 Characteristic pattern of alternate fallow and wheat strips where land is summer fallowed for wheat production. 76 SOIL WATER MANAGEMENT

FIGURE 6.4 Changes in soil water during a summer fallow period for wheat production. face greatly exceeds the water movement upward work perfectly, some evaporation occurs each from moist soil on a hot summer day. As a result, time the soil surface is wetted and dried, and the immediate surface of the soil becomes air dry some runoff occurs. A good estimate is that about within a few hours and the hydraulic conductivity 25 percent of the rainfall during the fallow period for liquid water approaches zero. This phase is will be stored in the soil for use in crop produc- associated with a sharp decline in water loss. tion. This extra quantity of water, however, has a Once a thin layer of surface soil is air dry, and great effect on yields, as is illustrated in Table 6.1. few water films bridge particles, water can move The frequency of yields over 1,345 or 2,690 kg/ from the underlying moist soil to and through the hectare (20 or 40 bushels/acre) was significantly surface as vapor flow. Vapor movement under increased by summer fallowing. Fallowing was these conditions is so slow as to be largely dis- also more effective at Pendleton, Oregon, where counted, unless cracks exist. The result is a cap- the maximum rainfall occurs in winter, unlike ping effect that protects the stored water from Akron, Colorado, and Hays, Kansas, where maxi- being lost by evaporation. When another rain oc- mum rainfall occurs in summer. Stored soil mois- curs, the surface soil is remoistened and addi- ture has been shown to be as effective as precipi- tional water moves through the previously moist- tation during the growing season for wheat ened soil to increase the depth of the recharged production. soil. Repetition of this sequence of events during a fallow period progressively increases the depth of recharged soil and the amount of stored soil Saline Seep Due to Fallowing water. The sequence of events during a fallow- Most of the world's spring wheat is grown in sub- cropping cycle is shown in Figure 6.4. humid or semiarid regions where winters are se- Many studies have shown a good correlation vere, such as the northern plains areas of United between water stored at planting time and grain States, Canada, and the Soviet Union. Low tem- yield. The fallowing system, however, is not 100 perature results in low evapotranspiration, which percent efficient. The capping feature does not increases the efficiency of water storage during WATER CONSERVATION 77

TABLE 6.1 Percentage Distribution of Wheat Yield Categories at Two Locations in the Great Plains and One Location in the Columbia River Basin

the fallow period. In fact, if a fallow period is spire more water per gram of plant tissue pro- followed by a wet year, surplus water may occur duced than those growing where the plant nutri- and percolate downward and out of the root zone. ents are in abundance. Since it has been shown Where surplus water encounters an impermeable that water loss (or use) is mainly dependent on layer, soil saturation occurs and water tends to the environment, any practice that increases the move laterally because of gravity above the imper- rate of plant growth will tend to result in produc- meable layer. This may create a spring (seepage tion of more dry matter over a given period of time spot) at a lower elevation. The evaporation of and per unit of water used. The data in Table 6.2 water from the area wetted by the spring results in show that the yield of oats increased from 86 to the accumulation of salts from the water, and an 145 kg/hectare (2.4 to 4.0 bushels/acre) for each area of salt accumulation called a saline seep is 2.5 centimeters of water used in conjunction with formed. The osmotic effect of soluble salt reduces more fertilizer. In humid regions, it has commonly the soil water potential, water uptake, and wheat been observed that fertilized crops are more yields. In some instances the soils become too drought resistant. This may be explained on the saline for plant growth, because of very low water basis that increased top growth results in in- potential. creased root growth and root penetration, so that Control of a saline seep depends on reducing the total amount of water consumed is greater. the likelihood that surplus water will occur. Man- Fertilizer use in the subhumid region may occa- agement practices to control saline seeps in- sionally decrease yields. If fertilizer causes a crop clude: (1) reduced fallowing frequency, (2) use of to grow faster in the early part of the season, the fertilizers to increase plant growth and water use, greater leaf area at an earlier date represents a and (3) the production of deep-rooted perennial greater potential for water loss by transpiration. If crops, such as alfalfa, to remove soil water. When no rain occurs, or no irrigation water is applied, alfalfa is grown and has dried out the soil, the plants could run out of water before harvest, caus- alfalfa grows very slowly, which indicates that it is ing a serious reduction in grain yield. once again time to begin wheat production. Because water use in crop production is mainly environmentally determined, fertilizer use and the doubling or tripling of yields has little effect on Effect of Fertilizers on Water the sufficiency of the water supply. In under- Use Efficiency developed countries, large increases in crop Plants growing in a soil that contains a relatively yields can be produced without creating a need small quantity of nutrients grow slower and tran- for significantly more water. Conversely, if water

78 SOIL WATER MANAGEMENT

TABLE 6.2 Oats Production per 2.5 Centimeters of Water Used

From Hanks and Tanner, 1952.

supplies become very limited, it may be desirable gravitational water from drainage ditches into to use less land and more fertilizer to obtain the higher canals for return to the sea. Now, 800 years same amount of total production with less water later, about one third of the Netherlands is pro- used. tected by dikes and kept dry with pumps and canals. Soil dewatering or drainage is used to lower the water table when it occurs at or near the soil surface, or to remove excess water that has SOIL DRAINAGE accumulated on the soil surface. The use of drainage is commonplace to dewater the soil in About A.D. 1200, farmers in the Netherlands be- the root zone of crops, to dewater the soil near came engaged in an interesting water control building foundations to maintain dry basements problem. Small patches of fertile soil, affected by and for the successful operation of septic sewage tides and floodwaters, were enclosed by dikes disposal systems, and to remove excess surface and drained. Windmills provided the energy to lift water (see Figure 6.5).

FIGURE 6.5 Dewatering, surface and subsurface drainage, is required to make this land suitable for both building construction and cropping. (Photograph courtesy Soil Conservation Service, USDA.) SOIL DRAINAGE 7 9

Water Table Depth Versus Air and This creates a zone above the capillary fringe that Water Content of Soil has a decreasing water content and increasing air If a well is dug and the lower part fills with water, content toward the soil surface. This capillary- the surface of the water in the well is the top of the affected zone, plus the capillary fringe, generally water table. Water does not rise in a well above ranges in thickness from several centimeters to the water table. In the adjacent soil, however, several meters. For all practical purposes, soil capillarity causes water to move upward in soil more than 2 meters above the water table is pores. A water-saturated zone is created in the soil scarcely affected by it. This means that most above the water table, unless the soil is so gravelly plants, even those in humid regions, do not use or stony that capillarity does not occur. The water- water from the water table. saturated zone above the water table is the capil- Generally, the soil above the capillary fringe lary fringe and is illustrated in Figure 6.6. will have a water content governed by the balance Soils typically have a wide range in pore sizes. between infiltration and loss by evapotran- Some pores are large enough so that they are spiration and percolation. Soils in deserts and air-filled above the capillary fringe, depending on semiarid regions typically have a permanently dry pore size and height above the capillary fringe. zone between the root zone and the water table.

FIGURE 6.6 Generalized relationship of the water table to the soil water zones, and the air and water content of soil above the water table. The capillary fringe is water saturated. Upward from the capillary fringe the water content of the soil decreases and the air content increases within the capillary fringe affected zone. 80 SOIL WATER MANAGEMENT

Benefits of Drainage Two conditions favoring the use of surface Root tips are regions of rapid cell divison and drainage are: (1) low areas that receive a large elongation, having a high oxygen demand. Roots amount of water from the surrounding higher of most economic crop plants do not penetrate land, and (2) impermeable soils that have an in- water-saturated soil or soil that is oxygen defi- sufficient hydraulic conductivity to dispose of the cient. The major purpose of drainage in agricul- excess water by movement downward and ture and forestry is lowering the water table to through the soil to drainage tubing. Surface increase the plant rooting depth. drainage is favored on the Sharkey clay soils along Some of the largest increases in plant growth the lower Mississippi River, because the soil is resulting from drainage occur in forests. Drainage slowly permeable and receives water from sur- on the Atlantic Coastal Plain increased the growth rounding higher land. Sugarcane, which is widely of pine from 80 percent to 1,300 percent (see grown and sensitive to poor aeration, is planted Table 6.3). Few management practices are as ef- on ridges to improve aeration in the root zone. fective as drainage for increasing tree growth. Ad- It is difficult to irrigate without applying excess ditional benefits of drainage in forests are easier water in many instances. Surface drainage ditches logging, with less soil disturbance, and easier site are used to dispose of excess water and to prevent preparation for the next crop. saturation of the soil on the low end of irrigated Drainage of wet soil also increases the length of fields. Surface drainage is also widely used along the growing season in regions with low soil tem- highways and in urban areas for surface water perature and frost hazard. Soils with a lower water control. Rapid removal of surface water from low content warm more rapidly in the spring. Seed areas of golf courses and parks is essential to germination is more rapid and roots grow faster. permit their use soon after a rain. The net result is a greater potential for plant growth. Subsurface Drainage Ditches for the removal of gravitational water can Surface Drainage be made quickly and inexpensively. Drainage Surface drainage is the collection and removal of ditches, however, require periodic cleaning and water from the surface of the soil via open ditches. are inconvenient for the use of machinery. There

TABLE 6.3 Effect of Drainage on Mean Annual Growth of Pine on Poorly Drained Soils of the Atlantic Coastal Plain

Adapted from Terry and Hughes, 1975. a Cubic meters per hectare. SOIL DRAINAGE 81 are also many situations in which ditches are not in containers is poor soil aeration caused by over- satisfactory, for example, water removal around watering. How can this be true, since most the basement walls of a building. flowerpots have a large drainage hole in the bot- Drain lines are installed in fields with trenching tom? Knowledge of changes in air and water con- machines. Perforated plastic tubing is very popu- tent of soil above the water table can help provide lar and is automatically laid at the bottom of a the answer. trench as the trenching machine moves across the Consider a flowerpot filled with soil that is wa- field (see Figure 6.7). Unless some unusual con- ter saturated. Water is allowed to drain out of the dition prevails, plastic tubing should be laid at large hole in the bottom of the pot. Surplus water least 1 meter deep to have a sufficiently low water will exit rapidly from the largest soil pores at the table between the drains to permit crops to de- top of the pot and air will be pulled into these pore velop an adequate root system, as illustrated in spaces. Gradually the zone of water saturation Figure 6.8. will move progressively downward. Shortly thereafter, the loss of water by gravity may essen- Drainage in Soil of tially stop. At this time, some of the soil at the Container-Grown Plants bottom may still be water saturated, if all the pores Many plants are grown indoors in containers. One in the soil are sufficiently small to attract water of the most common problems of growing plants more strongly than gravity. This creates a "capillary fringe" in the bottom of the pot. Then, the air and water content of the soil above the saturated FIGURE 6.7 Installation of perforated plastic tubing soil at the bottom is analogous to soil above the to remove water from a field during the wet season. The plastic tubing carries water to an outlet where saturated capillary fringe. Compare the drawing in the water is disposed. (Photo courtesy Soil Figure 6.9 with Figure 6.6 and note, in particular, Conservation Service, USDA.) the decrease in water and increase in air with distance upward in the soil above soil that is water saturated. If all soil pores are very small, all the soil in a flowerpot could remain saturated after watering. A good soil for container-grown plants has many pores too large to retain water against gravity. After wetting and drainage, these large pores will become air filled, even at the bottom of the container. Special attention must be given to soil used for container-grown plants. Ordinary loamy field or garden soils are unsatisfactory. A mix of one- half fine sand and one-half sphagnum peat moss, by volume, is recommended by the University of California to produce a mix with excellent physical properties for plant growth. When sand and soil are mixed together, the small soil particles fill the spaces between the sand particles, producing a low-porosity mix with few large aeration pores. Placing large gravel in the bottom of a flowerpot or in a hole dug outdoors will create a barrier to the downward flow of water from an overlying finer-textured soil layer or soil mix. The gravel 82 SOIL WATER MANAGEMENT

FIGURE 6.8 The effect of drainage lines in lowering the water table. The benefit of drainage is first evident directly over the drainage lines, and it gradually spreads to the soil area between them. acts in the same manner as a large hole in a imately 11 percent of the world's cropland is irri- container that is filled with a soil mix. A saturated gated. Some of the densest populations are sup- soil zone tends to form above the gravel layer, ported by producing crops on irrigated land, as in whatever the depth of the gravel. the United Arab Republic (Egypt), where 100 percent of the cropland is irrigated. This land is located along the Nile River. More than 12 percent of the cropland, or about 24,000,000 hectares I RRIGATION (61,000,000 acres) are irrigated in the United States. Irrigation is extremely important in in- Irrigation is an ancient agricultural practice that creasing soil productivity throughout the world. was used 7,000 years ago in Mesopotamia. Other ancient, notable irrigation systems were located in Egypt, China, Mexico, and Peru. Today, approx- Water Sources Most irrigation water is surface water resulting from rain and melting snow. Many rivers in the FIGURE 6.9 Illustration of conditions in a flowerpot world have their headwaters in mountains and after a thorough watering and drainage. At the bottom of the pot, the soil is saturated. Soil-air flow through arid and semiarid regions where the content increases and water content decreases with water is diverted for irrigation. Examples include distance upward above the water-saturated layer. The the Indus River, which starts in the Himalaya soil pore spaces operate in a manner similar to a Mountains, and the numerous rivers on the west- series of capillary tubes of varying sizes. ern slope of the Andes Mountains, which flow through the deserts of Chile and Peru to the Pa- cific Ocean. Much of the water in these rivers comes from the melting of snow in the high mountains. In fact, the extent of the snowpack is measured in order to predict the stream flow and amount of water that will be available for irrigation the next season. Many farmers who use well water have their own source of irrigation water. About 20 percent of the water used for irrigation in the United States comes from wells.

Important Properties of Irrigated Soils Soils well suited for irrigation have an intermediate texture to a depth of I to 2 meters and without compact or water-impermeable layers. I RRIGATION 83

Loamy soils that are underlain by coarse sands Furrow Irrigation Furrow irrigation is similar and gravel have greater retention of water in the to flood irrigation, the difference being that the overlying soil than soils that are loamy-textured water distribution is down furrows instead of be- throughout. The texture, together with the struc- ing allowed to flood over the land. In furrow irri- ture, should result in an infiltration rate of about gation, which is used for row crops, water is dis- 0.5 to 8 centimeters per hour and a moderate tributed down between the rows. Furrows that water-storage capacity. The soil should not have have a slight grade or slope are made across the an injurious soluble salt content. field. The slope of the furrow bottoms depends on soil infiltration rate, which should be sufficient to distribute water along the entire length of the fur- Water Application Methods row (see Figure 6.11). Furrow irrigation is unsatis- Choice of various methods of applying irrigation factory on sand soils, because of the very high water is influenced by a consideration of: (1) in- infiltration rate and limited distance of water flow filtration rate, (2) slope and general nature of the down the furrows. soil surface, (3) supply of water and how it is Many methods are used to distribute water into delivered, (4) crop rotation, and (5) seasonal rain- the furrows. Gated pipe is shown in Figure 6.11, fall. The methods of distributing water can be and the use of siphon tubes is shown in Figure classified as flood, furrow, subsurface, sprinkler, 6.12. Siphon tubes require extensive manual and drip or trickle. labor. A major advantage of flood and furrow irrigation is that the water is distributed by gravity, so water distribution costs tend to be low. Flood Irrigation Flood irrigation floods the land by distributing water by gravity. Water is let into the upper end of the field, usually from a large Sprinkler Irrigation Sprinklers are widely used ditch. Sometimes the water is distributed into ba- to irrigate lawns. Sprinkler systems are versatile sins, which is a common practice for orchards, and have some special advantages. On sands, pastures, and some grain crops, including rice where high infiltration rates permit water to flow (see Figure 6.10). Flood irrigation can easily over- only a short distance down a furrow, sprinklers water some parts of; a field while leaving some are essential to spread water uniformly over the parts underirrigated. field. On an uneven land surface, expensive land

FIGURE 6.10 Flood irrigation of grapes in the Central Valley in California. 84 SOIL WATER MANAGEMENT

FIGURE 6.11 Planned land with proper grade to give uniform distribution of water down the rows. Gated pipe is being used to distribute water into the furrows. leveling and grading can be avoided by the use of pivot sprinklers are clearly visible from the air. sprinklers. The portable nature of many sprinkler Their use on many rolling and sandy soils has systems makes them ideally suited for use where greatly increased the production of corn and other irrigation is used to supplement natural rainfall. grain crops on the Great Plains. In some areas, When connected to soil water-measuring appa- however, underground water reservoirs are being rati, sprinkler systems can be made automatic. depleted, and some irrigated areas are again be- Large self-propelled sprinkler systems, which ing used as grazing land and for fallowed wheat pivot in a large circle around a well, have become production. popular. Such systems can irrigate most of the Sprinkler irrigation modifies the environment land in a quarter section (160 acres, or 65 hec- by completely wetting plant leaves and the soil tares). In the western part of the United States, surface and by affecting the surrounding air tem- large green circular irrigated areas produced by perature. The high specific heat of water makes

FIGURE 6.12 Use of siphon tubes to transfer water from the head ditch into furrows. IRRIGATION 85 sprinkling an effective means of reducing frost tages include adaptability of the system to very hazard. Sprinkler irrigation has been used for frost steep land, where other methods are unsuited, protection in such diverse situations as for winter and the ability to maintain a more uniform soil vegetables in Florida, strawberries in Michigan, water potential during the growing season. and citrus in California. Potatoes develop an irregular shape and become less desirable if tubers alternately grow rapidly and slowly because of large changes in Subsurface Irrigation Subirrigation is irrigat- available water during the period of tuber ing by water movement upward from a water table development. located some distance below the soil surface. Some areas are naturally subirrigated where water tables are within a meter or two of the soil sur- Rate and Timing of Irrigation face. This occurs in the poorly drained areas of An ideal application of water would be a sufficient the Sandhills in north-central Nebraska. Artifical quantity to bring the soil in the root zone to field subirrigation is practiced in the Netherlands, capacity. More water may result in waterlogging of where subsurface drainage lines are used to drain a portion of the subsoil or in the loss of water by soils in the wet seasons and then used during drainage. If water percolates below the root zone drought for subirrigation. The subirrigation de- over a period of many years, the water may ac- pends on the use of drainage ditches and pumps cumulate above an impermeable layer and pro- to create and control a water table. Subirrigation duce a perched water table. Conversely, unless works well on the nearly level Florida coastal enough water is applied to result in some drain- plains. The sandy soils have high saturated hy- age, it is difficult to remove the salt in the irriga- draulic conductivity, and the natural water table is tion water and prevent the development of soil 1 meter or less from the soil surface much of the salinity. time. An accepted generalization is that the time to Subirrigation is more efficient in the use of wa- irrigate is when 50 to 60 percent of the plant- ter than other systems, and is adapted only for available water in the root zone has been used. special situations. In arid regions, the upward Potentiometers are used to measure the matric movement of water and its evaporation at the soil potential in various parts of the root zone, and the surface results in salt accumulation. Subirrigation timing of irrigation is based on water potential works best where natural rainfall leaches any salts readings. Computer programs are available for that may have accumulated. irrigation scheduling that are based on maintaining a record of gains and losses of soil water. Farmers commonly irrigate according to the ap- Drip Irrigation Drip irrigation is the frequent or pearance of the crops. Such crops as beans, cot- almost continuous application of a very small ton, and peanuts develop a dark-green color un- amount of water to a localized soil area. Plastic der moisture stress. Some species of lawn grasses hoses about 1 or 2 centimeters in diameter, with turn to an almost black-green color when wilting. emitters, are placed down the rows or around In most instances, crops should be irrigated be- trees. Only a small amount of the potential root fore marked wilting occurs. When irrigation water zone is wetted. Roots in the localized area can is in short supply, it may be better to curtail water absorb water rapidly because a high water poten- use early in the season (delay irrigation), so that tial, or water availability, is maintained. A major sufficient water is available during the reproduc- advantage of drip irrigation is the large reduction tive stages of plant growth, when water stress is in the total amount of water used. Other advan- the most injurious to crop yields. 86 SOIL WATER MANAGEMENT

Water Qualit Medium salinity hazard water (C2) can be used Water in the form of rain and snow is quite pure. if a moderate amount of leaching occurs. Plants By the time the water has reached farm fields, with moderate salt tolerance usually can be however, the water has picked up soluble materi- grown without special practices for salinity als (salts) from the soils and rocks over and control. through which the water moved. Four important High salinity hazard water (C3) cannot be used factors in determining water quality are: (1) total on soils with restricted drainage. Even with ade- salt concentration, (2) amount of sodium relative quate drainage, special management for salinity to other cations, (3) concentration of boron and control may be required. Plants with good salt other toxic elements, and (4) the bicarbonate tolerance should be selected. concentration. Very high salinity hazard water (C4) is not suitable for irrigation under ordinary conditions, but may be used occasionally under very special cir- Total Salt Concentration An acre-foot of water cumstances. The soil must be permeable and weighs about 2,720,000 pounds. If it contained have adequate drainage. Excess irrigation water 1 ppm of salt, it would contain 2.72 pounds of salt. must be applied to bring about considerable Water containing 735 ppm of salt would contain 1 leaching of salt, and very salt-tolerant crops ton of salt per acre-foot of water. An annual appli- should be selected. cation of 4 acre-feet of irrigation water containing 735 ppm of salt would involve adding salt equal to 4 tons per acre each year. Since such water would Sodium Adsorption Ratio The water quality be of good quality, there is great potential for classification in Figure 6.13 is also based on the developing sufficient soluble salt content in soils sodium hazard, along the left vertical side of the so as to seriously reduce soil productivity. The figure. The sodium hazard is based on the sodium longtime use of soils for irrigation necessitates adsorption ratio (SAR), which is the amount of some provision for the leaching and removal of sodium relative to other cations. salt in drainage water in order to maintain a favor- The negative electrical charge of clay and hu- able salt balance in the soil, where the salts are mus particles is neutralized by cations that are not naturally leached out of the soil. adsorbed and balance the negative charge. This The total salt concentration, or salinity, is ex- charge, which is discussed in Chapter 10, is differ- pressed in terms of the electrical conductivity of ent and much stronger than the electronegativity the water, and it is easily and precisely deter- that attracts water molecules. When 15 percent or mined. The electrical conductivity of irrigation more of the charge is neutralized by sodium water is expressed as decisiemens per meter (dS/ (Na'), clay and humus particles tend to disperse m);in earlier literature as millimhos per cen- and decrease the soil's hydraulic conductivity. timeter. Four water quality classes have been es- The SAR expresses the relative amount of sodium tablished with conductivity ranging from 0.1 to in irrigation water relative to other cations and the more than 2.25 dS/m, as shown in Figure 6.13. likelihood that the irrigation water will increase The salinity hazard ranges from low to very high. the amount of adsorbed sodium in the soil. The four salinity classes of water have the fol- The SAR of irrigation water is calculated as lowing general characteristics. Low salinity water follows: (C1) can be used for irrigation of most crops, with (sodium) little likelihood that soil salinity will develop. SAR = 1 1² Some leaching is required, but this occurs under (calcium + magnesium) normal irrigation practices, except in soils with The ion concentrations, denoted by parentheses, extremely low hydraulic conductivity. are expressed in moles per liter. The classifi- IRRIGATION 87

FIGURE 6.13 Diagram for the classification of irrigation waters. (Data from USDA Handbook 60, 1954.) cation of irrigation waters with respect to SAR is toxic. Plants have a considerable range in toler- based primarily on the effect of the exchangeable ance. The occurrence of boron in toxic concentra- sodium on the physical condition of the soil. Irri- tion in certain irrigation waters makes it necessary gation water with SAR values of 18 to 26 has a high to consider this element in assessing water qual- sodium hazard and above 26 the sodium hazard is ity. The permissible limit of boron in several very high. The four SAR classes combine with the classes of irrigation water is given in Table 6.4, four classes of salinity hazard to give 16 classes of and the relative tolerance of some plants is given water as shown in Figure 6.13. in Table 6.5.

Boron Concentration Boron is essential to Bicarbonate Concentration In waters contain- plant growth, but the amount needed is very ing high concentrations of bicarbonate ion - small. A small amount of boron in soil may be (HC0 3 ), calcium and magnesium tend to pre- SOIL WATER MANAGEMENT

TABLE 6.4 Permissible Limits of Boron for Several Classes of Irrigation Waters in Parts per Million Sensitive Semitolerant Tolerant Boron Class Crops Crops Crops 1 <0.33 <0.67 <1.00 2 0.33-0.67 0.67-1.33 1.00-2.00 3 0.67-1.00 1.33-2.00 2.00-3.00 4 1.00-1.25 2.00-2.50 3.00-3.75 5 >1.25 >2.50 >3.75

From Agriculture Handbook 60, USDA, 1954. cipitate as carbonates as the soil solution be- In making an estimate of water quality, the ef- comes more concentrated. This reaction does not fect of salts on both the soil and the plant must be go to completion under ordinary circumstances, considered. Various factors such as drainage and but when it does proceed, the concentrations of soil texture influence the effects on soil. The finer calcium and magnesium are reduced and the rel- the size of clay particles and the greater the ten- ative proportion of sodium in the soil solution is dency for clay particles to expand and contract increased. This results in an increase in the SAR with wetting and drying, the greater is the effect of and a decrease in water quality. a given SAR on dispersion of clay particles and

TABLE 6.5 Tolerance of Plants to Boron (In each Group, the Plants First Named are Considered as Being More Tolerant, and the Last Named More Sensitive)

From Agriculture Handbook 60, USDA, 1954. IRRIGATION 89 reduction in hydraulic conductivity. The ultimate 5. Over 16: Only a few very salt-tolerant crops effect on the plant is the result of these and other yield satisfactorily. factors that operate simultaneously. Conse- The salt-tolerance figures of crops given in Ta- quently, no method of interpretation is absolutely ble 6.6 are arranged according to major crop divi- accurate under all conditions, and interpretations sions. Within each group the crops are listed in are based commonly on experience. order of decreasing salt tolerance. The EC e values given represent the salinity level at which 10, 25, Salt Accumulation and Plant Response and 50 percent decreases in yields may be ex- Soil salinity caused by the application of irrigation pected, as compared with yields on nonsaline water usually is not a problem in humid regions soils under comparable growing conditions. Stud- where rainfall leaches the soil some time each ies of the effects of salt on plants have shown year. Most irrigation, however, is done in low rain- significant differences in varieties of cotton, bar- fall areas where leaching is limited and salt tends ley, and smooth brome, whereas variety differ- to accumulate in irrigated soils. Today, crop pro- ences were of no consequence for green beans, duction is reduced on 50 percent of the world's lettuce, onions, and carrots. irrigated land as a result of salinity or drainage The effect of salts on plants is mainly indirect, problems. that is, the effect of salt on the osmotic water Saline soil contains sufficient salt to impair potential. Decreasing water potential due to salt plant growth. The salts are mainly chlorides, bi- reduces the rate of water uptake by roots and carbonates, and sulfates of sodium, potassium, germinating seeds. Thus, an increase in soil salin- calcium, and magnesium. The best method for ity produces the same net effect on water uptake assessing soil salinity is to measure the electrical as that produced by soil drying. In fact, the effects conductivity of the saturated soil extract, the EC e . of decreases in osmotic and matric water poten- This procedure involves preparing a water- tials appear to be additive. saturated soil paste by adding distilled water while stirring the soil until a characteristic end point is reached. A suction filter is used to obtain a sample of the extract for making an electrical Salinity Control and conductivity measurement. The saturated-extract Leaching Requirement electrical conductivity is directly related to the For permanent agriculture, there must be a favor- field moisture range from field capacity to the able salt balance. The salt added to soils in irri- permanent wilting point; this is a good parameter gation water must be balanced by the removal of for evaluating the effect of soil salinity on plant salt via leaching. The fraction of the irrigation growth. water that must be leached through the root zone leaching re- Salinity effects on plants, as measured by the to control soil salinity is termed the Assuming that the goal is to irri- electrical conductivity of the saturated extract, quirement (LR). gate a productive soil with no change in salinity, ECe , in decisiemens per meter, dS/m, are as follows: and where soil salinity will not be affected by leaching from rainfall or other factors, the leach- 1. 0-1: Salinity effects mostly negligible. ing requirement is the ratio of the depth of 2. 2-4: Yields of very salt-sensitive crops may be drainage water, D dw , to the depth of irrigation restricted. water, D 1 : 3. 4-8: Yields of many crops restricted. = Data 4. 8-16: Only salt-tolerant crops yield satisfac- LR torily. D;W 90 SOIL WATER MANAGEMENT

TABLE 6.6 Salt Tolerance of Crops Expressed as the EC, at 25° C for Yield Reductions of 10, 25, and 50 Percent, as Compared to Growth on Normal Soils IRRIGATION 91

TABLE 6.6 (continued)

Adapted from Ag. Ifai B. N. 283 ad 292 and Wee Feiie Hadb, 1975.

LR, under the conditions specified (no change in lights a major disadvantage of drip irrigation: the soil salinity over time), is also equal to method is not suitable for application of large amounts of water. Leaching of salts without pro- EC;, LR = vision for drainage in upland soils, as on the high EC dW plains of Texas, and other irrigated areas, results If the electrical conductivity of the drainage water in salt being deposited below the root zone. Most

(ECdW) that can be tolerated without adversely of the irrigated land in arid regions is located in affecting the crop is 8 dS/m: basins or alluvial valleys containing stratified soil materials. Long and continued leaching for salt ECiw LR - control commonly results in the accumulation of 8 dS/m surplus water above impermeable layers or the For irrigation water with an electrical conductivity formation of perched water tables. When water

(ECi w) of 1, 2, and 3 dS/m, LR = 0.13, 0.25, and tables rise to within 1 to 2 meters of the soil 0.38, respectively. Of the irrigation water applied surface, water moves upward by capillarity at a to the soil, 13, 25, and 38 percent, respectively, rate sufficient to deposit salt on top of the soil represent the amounts of the water that must be from the evaporation of water, as shown in Figure leached through the soil. Under these conditions, 6.14. It is essential in these cases that subsurface the salt in the irrigation water is equal to the salt in drainage systems be installed to remove drainage the drainage water and the salt content of the soil water and maintain deep water tables. Conse- remains unchanged. When the irrigation need is quently, salinity control in arid regions is depen- 10 centimeters, the total water application needed dent on drainage. is calculated as follows for an LR of 25 percent: Archaeological studies in Iraq showed that the Sumerians grew almost equal amounts of wheat LR = 10 cm + 0.25 (10 cm) = 12.5 cm and barley in 3500 B.C. One thousand years later, The need for leaching soils to control salt high- only one-sixth of the grain was the less salt- 92 SOIL WATER MANAGEMENT

FIGURE 6.14 Water tables (water-saturated soil) a short distance below the soil surface result in the upward movement and evaporation of water and the accumulation of salt at the soil surface. The process results in the development of saline soil.

tolerant wheat; by 1700 B.C., the production of lowed, important differences can exist in the salt wheat was abandoned. This decline in wheat pro- content of soil that is immediately under the irri- duction and increase in salt-tolerant barley coin- gation furrow and the tops of beds. Downward cided with soil salinization. Records show wide- water movement under the furrow leaches the soil spread land abandonment, and that salt and produces low salinity. At the same time, water accumulation in soils was a factor in the demise moves to the top of the beds and salt is deposited of the Sumerian civilization. as water evaporates. Important differences in salt Even though good irrigation practices are fol- concentrations are produced (see Figure 6.15).

FIGURE 6.15 Salt accumulation on the ridges resulting from the upward movement and evaporation of water in a cotton field. Salt content under the furrows is less than on the ridges due to leaching under the furrows. (Photo courtesy Soil Conservation Service, USDA.) IRRIGATION 93

Effect of Irrigation on River Soil permeability, or hydraulic conductivity, has Water Quality not been adversely affected by adsorbed sodium. About 60 percent of the water diverted for irri- Reclamation of saline soils requires the re- gation is evaporated or consumed. The remainder moval of salt by leaching and maintenance of the appears as irrigation return flow (which includes water table far enough below the soil surface to drainage water) and is returned to rivers for dis- minimize upward capillary water flow. Important posal. As a consequence, the salt concentration considerations are soil hydraulic conductivity and of river water is increased. A 2- to 7-fold increase an adequate drainage system. It is not economical in salt concentration is common for many rivers. to reclaim some saline soils that have high clay Along the Rio Grande in New Mexico, water diver- content and a very low hydraulic conductivity. ted at Percha Dam for Rincon Valley irrigation had Several practices are recommended for using an average of 8 milliequivalents per liter of salt saline soils. The soil should not be allowed to dry from 1954 to 1963. Salt content increased to 9 at completely, because the osmotic potential retards Leasburg, 13 at the American Diversion Dam, and water uptake. Sufficient water should be applied 30 milliequivalents per liter at the lower end of El with each irrigation to result in leaching of some Paso Valley. Rivers serve as sinks for the deposi- salt in drainage water. Emphasis must be placed tion of salt in drainage water from irrigated land. on the selection and growth of salt-tolerant crops. The Salton Sea, whose surface is below sea level, Considerable research effort is being directed to- serves as a salt sink for the Imperial Valley in ward the development of cultivars with greater southern California. salt tolerance.

Nature and Management of Saline and Sodic Soils Sodic soils have been called alkali Sodic Soils soils. Sodic soils are nonsaline and have an ad- The development of saline soils is a natural sorbed or exchangeable sodium percentage process in arid regions where water tables are (ESP) of 15 or more. The adsorbed cations (so- near the soil surface. Large areas have become dium, calcium, magnesium, and potassium) are saline from irrigation and from saline seeps as a considered exchangeable, because they compete result of summer fallowing. When sodium is an for adsorption on the negatively charged soil col- important component of the salt, a significant loids (clay and humus), are in motion, and ex- amount of adsorbed sodium may occur and may change sites with each other. The sodium ad- disperse soil colloids and develop undesirable sorption ratio (SAR) of a soil-saturated paste physical properties. Such soils are sodic. The extract is about equal to the ESP. A SAR of 13 or quantity, proportion, and nature of salts that are more is the same as ESP of 15 or more. The pH is present may vary in saline and sodic soils. This usually in the range of 8.5 to 10.0. Sodium disso- gives rise to three kinds of soils: saline, sodic, and ciates from the colloids and small amounts of saline-sodic. sodium carbonate form. Deflocculation or dispersion of colloids occurs, together with a breakdown of the soil structure as shown in Fig- Saline Soils Saline soils contain enough salt to ure 6.16. This results in a massive or puddled soil impair plant growth. White crusts of salt fre- with low water infiltration and very low hydraulic quently accumulate at the soil surface. The elec- conductivity within the soil. The soil is difficult to trical conductivity of the saturated-soil extract is 4 till, and soil crusting may inhibit seedling emer- or more dS/m. The pH is 8.5, or less, and less than gence. Organic matter in the soil disperses along 15 percent of the adsorbed cations are sodium. with the clay, and the humus coats soil particles 94 SOIL WATER MANAGEMENT

Saline-Sodic Soils Saline-sodic soils are characterized by salinity and 15 percent or more of exchangeable sodium. As long as a large quantity of soluble salts remains in the soil, the exhange- able sodium does not cause dispersion of colloids. The high concentration of ions in the soil solution keeps the colloids flocculated. If the soluble salts are leached out, however, colloids may disperse with an accompanying breakdown of soil structure and loss of water permeability. The management of these soils is a problem until the excess soluble salt has been removed and the exchangeable sodium level is reduced to below 15 percent. If the soluble salts are removed by leaching without lowering the exchangeable sodium, saline-sodic soils may be converted in sodic soils. Consequently, gypsum must be added to the soil at the time leaching of salts is initiated.

FIGURE 6.16 Distintegrated soil structure in an irrigated field caused by high adsorbed sodium. (Photo courtesy Soil Conservation Service, USDA.) WASTEWATER DISPOSAL The billions of gallons of wastewater produced to give a black color. Sodic soils may develop daily in the United States create a need for envi- when the leaching of a saline soil results in high ronmentally acceptable water disposal methods. exchangeable sodium and low exchangeable cal- The three major wastewater disposal techniques cium and magnesium. are: (1) treatment and discharge into surface wa- The basis for treatment of sodic soil is the ters, (2) treatment and reuse, and (3) land dis- replacement of exchangeable sodium with cal- posal via septic tanks and spray irrigation. Major cium and the conversion of any sodium carbonate consideration will be given to land disposal of into sodium sulfate. Finely ground gypsum (Ca- sewage effluent from septic systems and the man- - - S04 21 1²0) is broadcast and mixed with the soil. agement of municipal wastewater that results in The calcium replaces or exchanges for the so- return of treated water to underlying aquifers for dium adsorbed on the negatively charged colloid reuse. surfaces. The amount of gypsum needed to replace the exchangeable sodium is the gypsum requirement. Disposal of Septic Tank Effluent The calcium sulfate also reacts with any so- People who reside beyond the limits of municipal dium carbonate that is present to form sodium sewer lines have used septic sewage disposal sys- sulfate and calcium carbonate. Then, leaching of tems for many years. The expansion of rural resi- the soil removes both the adsorbed sodium and dential areas, and the development of summer the sodium that was in the carbonate form, as homes near lakes and rivers have greatly insoluble sodium sulfate. Gradually, the ex- creased the need for waste disposal in rural areas. changeable sodium percentage decreases to less The sewage enters a septic tank, usually con- than 15 percent, and the soil's structure and per- structed of concrete, where solid material is di- meability improve. gested or decomposed. The liquid portion, or ef-

WASTEWATER DISPOSAL 95

fluent, flows out of the top of the septic tank and holes should be uniformly spaced over the into drainage lines of a filter field. The drainage area selected for the filter field. Roughen lines are laid in a bed of gravel into which the the sides of the holes to remove smeared soil effluent seeps and drains through the soil, as illus- that might interfere with water entry into the trated in Figure 6.17. soil. Add 5 centimeters of sand or fine gravel The major factor influencing the suitability of to the bottom of the holes to reduce the soil's soil for filter field use is the water-saturated hy- tendency to seal. draulic conductivity. The hydraulic conductivity 2. Pour enough water into each hole so that the of sands may be too high, because the sewage water is at least 30 centimeters deep. Add effluent may move so rapidly that disease organ- water as needed to keep the water depth to 30 isms are not destroyed before shallow-water sup- centimeters for at least 4 hours-preferably plies become contaminated. Soils with low hy- overnight. This wets the soil and produces draulic conductivity are not suited for septic drain results representative of the wettest season of fields, because the sewage effluent may saturate the year. soil and contaminate the surface soil. 3. The next day, adjust the water level in the test Most local regulations require that trained per- holes so that the water is 30 centimeters deep. sonnel make a percolation test before approving Measure the drop in water level over a 30- the installation of a septic system. The steps for minute period and calculate the percolation making a soil percolation test (perc test) are as rate. follows: 4. Soils with a percolation rate less than 2.5 centimeters (1 in.) per hour are unsuited for 1. Dig six or more test holes about 15 cen- filter fields because of the danger of satura- timeters in diameter and to the depth of the tion of the soil above the filter bed with efflu- proposed drainage lines or seepage bed; the ent and the leakage of effluent from the soil

FIGURE 6.17 The layout of a septic tank and filter field for disposal of wastewater (effluent) through the soil. 96 SOIL WATER MANAGEMENT

surface. Soils with percolation rates greater Land Disposal of Municipal Wastewater than 25 centimeters (10 in.) per hour are un- A relatively small community of 10,000 people suited for filter fields because rapid move- may produce about a million gallons of waste- ment of effluent might pollute groundwater. water, or sewage effluent, per day from its sewage treatment plant. The effluent looks much like ordi- Within the acceptable range of percolation nary tap water; when chlorinated, it is safe for values, increasing percolation rates result in de- drinking. Discharging the effluent into a nearby creasing seepage-bed area. The more people liv- stream does not create a health hazard. The ef- ing in a dwelling, the more effluent for disposal. A fluent, however, is enriched with nutrients, so graph is used to determine the area of seepage plant growth may be greatly increased in streams bed needed based on soil percolation rate and and lakes where the effluent is discharged. Weed number of house occupants. growth in lakes and streams may reduce fish pop- Several other factors are considered in selec- ulations and interfere with boating and tion of filter field or seepage-bed sites. They swimming. To combat these problems, research- should be located at least 50 feet from any stream, ers at the Pennsylvania State University designed open ditch, or other watercourse into which ef- experiments to use the soil as a living filter to fluent might escape, and the land slope should be remove the nutrients in sewage effluent. The wa- less than 10 percent. Further, septic seepage beds ter was applied with sprinklers, as shown in Fig- should never be installed in poorly drained soils ure 6.18. or where soils become seasonally saturated with As the effluent water percolates through the water (see Figure 6.5). There is danger that shal- soil, nutrients are absorbed by plant roots and low groundwater may be contaminated with dis- micoorganisms. More water is applied than lost ease organisms and that water may seep out of the by evapotranspiration and the excess water per- soil surface and contaminate work or play areas. colates from the bottom of the soil and is added to

FIGURE 6.18 Application of wastewater (effluent) in a forest with a sprinkler system in winter at Pennsylvania State University. (Photo courtesy USDA.) PRESCRIPTION ATHLETIC TURF 97 the underground water reservoir (aquifer). A com- nomically and environmentally sound. An exam- munity of 10,000 people would need 50 hectares ple of a wastewater land disposal system that uses (about 125 acres) and a weekly application of 5 nutrients for plant growth and return of water to an centimeters of water to dispose of their effluent. underlying aquifer is shown in Figure 6.19. Under these conditions about 80 percent of the effluent water will contribute to aquifier recharge and about 20 percent will be evapotranspired. In PRESCRIPTION ATHLETIC TURF the Pennsylvania State University experiment, an almost complete removal of the nitrogen and Prescription athletic turf (PAT) is a system of pro- phosphorus in the effluent occurred-the two nu- ducing vigorous turf for use on athletic fields. It trients most closely tied to eutrophication. Fur- combines several aspects of water management, thermore, tree and crop growth on the land were including irrigation and drainage, and was devel- greatly stimulated. oped at Purdue University, where football is Today, more and more cities are utilizing land played on natural grass turf. disposal of sewage effluent. In some areas, the The existing turf area is excavated to a depth of trees produced in the disposal area are about 18 inches and the soil is removed. A plastic chipped and used as fuel to dry and process the liner is laid on the newly exposed surface, 2-inch solid sewage sludge. Marketing the sludge as a diameter slitted plastic drain lines are laid on top fertilizer results in a considerable cost saving in of the plastic, and sensors for measuring water sewage disposal and conservation of nutrients. potential and salinity are installed. The main Additional benefits include the existence of a pro- features of the PAT system are shown in Figure tected forested or green zone near the community 6.20. The sand layer forms the base for a 2-inch- and recharge of underground water aquifers. Such thick surface layer of sand, peat, and soil. The wastewater management systems can be eco- water potential sensors maintain a thin layer of

FIGURE 6.19 Sewage-treatment system designed to apply wastewater to the land for crop and forest production. The water in excess of evaporation percolates to the water table and contributes to the groundwater (aquifer). (Modified from Woodwell, 1977.) 98 SOIL WATER MANAGEMENT

FIGURE 6.20 Diagram of layout for prescription athletic turf, PAT. The drainage lines remove excess water in wet season and are used for subirrigation in dry seasons. Sensors monitor water and salinity. (Data provided by Prescription Athletic Turf, Inc.)

water above the plastic liner. Grass roots are sup- content of soil, a decrease in the water content, plied by water that moves slowly upward by capil- and a decrease in the matric water potential. larity. During periods of rain, pumps pull excess Irrigation water contains soluble salts and water out of the soil so that the field is playable some provision (e.g., drainage) must be made to with firm traction regardless of the weather. The maintain a favorable salt balance to keep irrigated salt sensors monitor the buildup of salts from soils permanently productive. Salt accumulation irrigation and fertilization. The sandy nature of the results in formation of saline soils. The accumu- soil resists compaction by traffic. lation of exchangeable sodium, greater than about 15 percent of the adsorbed cations, results in the formation of sodic soils. High sodium saturation results in a deterioration of soil structure SUMMARY and a loss of soil water permeability. Sodic soils The three approaches to water management on can be improved by applying gypsum and then agricultural fields include the conservation of nat- leaching the soil. ural precipitation in arid and subhumid regions, Wastewater (sewage effluent) disposal on the the removal of water (drainage) from wetlands, land results in removal of nutrient ions and return and irrigation to supplement natural precipi- of water to the underlying aquifers. tation. Prescription athletic turf is an artifical system Protection of the land surface by vegetation and for the production of vigorous grass on athletic plant residues shields the soil surface from rain- fields by precise control of water, aeration, drop impact and encourages water infiltration. drainage, salinity, and nutrient supply. Summer fallowing results in storage of water for crop production at a later time. Fertilizer use may increase the efficiency of water use by plants by increasing the rate of plant growth. Removal of water by drainage provides an aer- REFERENCES ated root zone for crops and dry soil for building Baker, K. F., ed. 1957. "The U. C. System for Producing foundations. Upward from the surface of a water- Healthy Container-Grown Plants." Ca. Agr. Exp. Sta. saturated soil layer, there is an increase in the air Manual 23. Berkeley. REFERENCES 99

Bender, W. H. 1961. "Soils Suitable for Septic Tank ity of Our Environment. AAAS Pub. 85, Washing- Filter Fields." Agr. Information Bull. 243. USDA, ton, D.C. Washington, D.C. Kemper, W. D., T. J. Trout, A. Segeren, and M. Bullock. Bernstein, L. 1964. "Salt Tolerance of Plants." Agr Infor- 1987. "Worms and Water." J. Soil and Water Conser- mation Bull. 283. USDA, Washington, D.C. vation. 42:401-404. Bernstein, L. 1965. "Salt Tolerance of Fruit Trees." Agr. Mathews, O. R. 1951. "Place of Summer Fallow in the Information Bull. 292. USDA, Washington, D.C. Agriculture of the Western States." USDA Cir. 886. Bower, C. A., 1974. "Salinity of Drainage Waters." Washington, D.C. Drainage in Agriculture. Am. Soc. Agron. Madison. Miller, D. E. and Richard O. Gifford. 1974. "Modification pp. 471-487. of Soil Crusts for Plant Growth," in Arizona Agr. Exp. Edminister, T. W. and R. C. Reeve. 1957. "Drainage Sta. Tech. Bull. 214. Problems and Methods." Soil. USDA Yearbook of Ag- Soil Improvement Comm. Calif. Fert. Assoc. 1985. riculture. Washington, D.C., pp. 379-385. Western Fertilizer Handbook. 7th ed. Interstate, Dan- Evans, C. E. and E. R. Lemon. 1957. "Conserving Soil ville, Ill. Moisture." Soil. USDA Yearbook of Agriculture. Terry, T. A. and J. H. Hughes. 1975. "The Effects of Washington, D.C. Intensive Management on Planted Loblolly Pine Hanks, R. J. and C. B. Tanner. 1952. "Water Consump- Growth on the Poorly Drained Soils of the Atlantic tion by Plants as Influenced by Soil Fertility. "Agron. J. Coastal Plain." Proc. Fourth North Am. Forest Soils 44:99. Conf. Univ. Laval, Quebec. Jacobsen, T. and R. M. Adams. 1958. "Salt and Silt in United States Salinity Laboratory Staff, 1969. "Diagnosis Ancient Mesopotamian Agriculture." Science. and Improvement of Saline and Alkali Soils." USDA 128:1251-1258. Agr. Handbook 60. Washington, D.C. Kardos, L. T. 1967. "Waste Water Renovation by the Woodwell, G. M. 1977. "Recycling Sewage through Land-A Living Filter." in Agriculture and the Qual- Plant Communities." Am. Scientist 65:556-562. CHAPTER 7

SOIL EROSION

Soil formation and soil erosion are two natural banks by water causes large masses of soil to fall and opposing processes. Many natural, undis- into the stream and be carried away. Very wet soil turbed soils have a rate of formation that is bal- masses on steep slopes are likely to slide slowly anced by a rate of erosion. Under these condi- downhill (solifluction). tions, the soil appears to remain in a constant Raindrops falling on bare soil detach particles state as the landscape evolves. Generally, the and splash them up into the air. When there is rates of soil erosion are low unless the soil surface little water at the soil surface, the water tends to is exposed directly to the wind and rainwater. The run over the soil surface as a thin sheet, causing erosion problem arises when the natural vegeta- sheet erosion. Rill andgully erosion are due to the tive cover is removed and rates of soil erosion are energy of water that has concentrated and is mov- greatly accelerated. Then, the rate of soil erosion ing downslope. As water concentrates, it first greatly exceeds the rate of soil formation and forms small channels-called rills-followed by there is a need for erosion control practices that greater concentrations of water. Large concentra- will reduce the erosion rate and maintain soil tions of water lead to gully formation. Some of the productivity. highest soil erosion rates occur at construction Erosion is a three-step process: detachment fol- sites where the vegetation has been removed and lowed by transport and deposition. The energy for the soil is exposed to the erosive effects of rain, as erosion is derived from falling rain and the sub- shown in Figure 7.1. The tendency of water to sequent movement of runoff water or the wind. collect into small rills that coalesce to form large channels and produce gully erosion is also shown in Figure 7.1. WATER EROSION Predicting Water Erosion Rates on Several types of water erosion have been iden- Agricultural Land tified: raindrop (splash), sheet, rill, and gully or The rate of soil erosion on agricultural land is channel. In addition, undercutting of stream affected by rainfall characteristics, soil erodibility,

100 WATER EROSION 101

These quantative data form the basis for predicting erosion rates by using the Universal Soil- Loss Equation (USLE) developed by Wischmeier and Smith. The USLE equation is

where A is the computed soil loss per unit area as tons per acre, R is the rainfall factor, K is the soil-erodibility factor, L is the slope-length factor, S is the slope-gradient factor, C is the cropping- management factor, and P is the erosion-control practice factor. The equation is designed to predict water erosion rates on agricultural land surfaces, exclusive of erosion resulting from the for- FIGURE 7.1 Removal of the vegetative cover mation of large gullies. increases water runoff and leaves soil unprotected from the erosive effects of the rain, resulting in accelerated soil erosion. As water concentrates in channels, its erosive power increases. (Photograph R = The Rainfall Factor courtesy Soil Conservation Service, USDA.) The rainfall factor (R) is a measure of the erosive force of specific rainfall. The erosive force, or available energy, is related to both the quantity slope characteristics, and vegetative cover and/or and intensity of rainfall. A 5-centimeter rain falling management practices. The first plots in the at 32 kilometers per hour (20 mph) would have 6 United States for measuring runoff and erosion, as million foot-pounds of kinetic energy. The tre- influenced by different crops, were established at mendous erosive power of such a rain is sufficient the University of Missouri in 1917 (see Figure 7.2). to raise an 18-centimeter-thick furrow slice to a Since 1930, many controlled studies on field plots height of 1 meter. The four most intense storms in and small watersheds have been conducted to a 10-year period at Clarinda, Iowa, accounted for study factors affecting erosion. Experiments were 40 percent of the erosion and only 3 percent of the conducted on various soils at many different loca- water runoff on plots planted with corn that were tions in the United States, using the same stan- tilled up and down the slope. Anything that dard conditions. For example, many erosion plots protects the soil from raindrop impact, such as were 72.6 feet long on 9 percent slopes and were plant cover or stones, protects the soil from ero- subjected to the same soil management practices. sion, as shown in Figure 7.3.

FIGURE 7.2 Site of the first plots in the United States for measuring runoff and erosion as influenced by different crops. The plots were established in 1917 at the University of Missouri. (Photograph courtesy Dr. C. M. Woodruff, University of Missouri.) 102 SOIL EROSION

The rainfall factor (R) is the product of the total kinetic energy of the storms times the maximum 30-minute intensity of fall and modified by any influence of snowmelt. Rainfall factors have been computed for many locations, with values ranging from less than 20 in western United States to 550 along the Gulf Coast of the southeastern United States. An iso-erodent map of average annual values of R is shown in Figure 7.4.

K = The Soil Erodibility Factor Soil factors that influence erodibility by water are: FIGURE 7.3 Columns of soil capped by stones that (1) those factors that affect infiltration rate, per- absorbed the energy of raindrop impact and meability, and total water retention capacity, and protected the underlying soil from erosion. (2) those factors that allow soil to resist disper- WATER EROSION 103 sion, splashing, abrasion, and the transporting point where the slope decreases to the extent that forces of rainfall and runoff. The erodibility factor deposition occurs, or a point where water runoff (K) has been determined experimentally for 23 enters a well-defined channel. Runoff from the major soils on which soil erosion studies were upper part of a slope contributes to the total conducted since 1930. The soil loss from a plot amount of runoff that occurs on the lower part of 72.6 feet long on a 9 percent slope maintained in the slope. This increases the quantity of water fallow (bare soil surface and with no crop running over the lower part of the slope, thus planted) and with all tillage up and down the increasing erosion more on the lower part of the slope, is determined and divided by the rainfall slope than on the upper part. Studies have shown factor for the storms producing the erosion. This that erosion via water increases as the 0.5 power is the erodibility factor. Values of K for 23 soils are of the slope length (L ° 5). This is used to calculate listed in Table 7.1. the slope length factor, L. This results in about 1.3 times greater average soil loss per acre for every doubling of slope length. LS = The Slope Length and Slope As the slope gradient (percent of slope) in- Gradient Factors creases (S), the velocity of runoff water increases, Slope length is defined as the distance from the which in turn increases the water's erosive power. point of origin of overland water flow either to a A doubling of runoff water velocity increases ero-

TABLE 7.1 Computed K Values for Soils

From Wischmeier and Smith, 1965. a Evaluated from continuous fallow. All others were computed from row crop data. 104 SOIL EROSION sive power four times, and causes a 32-time in- all interrelated crop cover and management vari- crease in the amount of material of a given parti- ables including type of tillage, residue manage- cle size that can be carried in the runoff water. ment, and time of soil protection by vegetation. Splash erosion by raindrop impact causes a net Frequently, different crops are grown in sequence downslope movement of soil. Downward slope or in rotation; for example, corn is grown one year movement also increases with an increase in the followed by wheat, which is then followed by a slope gradient. Combined slope length and gradi- hay or meadow crop. This would be a three-year ent factors (LS) for use in the soil-loss prediction rotation of corn-wheat-meadow (C-W-M). The equation are given in Figure 7.5. benefit of a rotation is that a year of high erosion, during the corn production year, is averaged with years of lower erosion to give a lower average C = The Cropping-Management Factor erosion rate, than if corn was planted every year. A vegetative cover can absorb the kinetic energy However, when corn is grown no-till, there may of falling raindrops and diffuse the rain's erosive be little soil erosion because of the protection of potential. Furthermore, vegetation by itself retains the soil by previous crop residues, as shown in a significant amount of the rain and slows the flow Figure 7.7. of runoff water. As a result, the presence or ab- Complicated tables have been devised for cal- sence of a vegetative cover determines whether culating the C factor. As an illustration, the C erosion will be a serious problem. This is shown factor for a 4-year rotation of wheat-alfalfa and in Figure 7.6 where erosion on plots 72.6 feet long bromegrass-corn-corn (W-AB-C-C) in central In- with an 8 percent slope had about 1,000 times diana with conventional tillage, average residue more erosion when in continuous corn as com- management, and average yields is 0.119. Some pared with continuous bluegrass. comparison values are as low as 0.03 for corn The C factor measures the combined effect of mulched in no-till with 90 percent of the soil

FIGURE 7.5 Slope length and gradient graph for determining the topographic factor, LS. WATER EROSION 105

fered by sod or closely growing crops in the system needs to be supported by practices that will slow runoff water, thus reducing the amount of soil carried. The most important of these practices for croplands are contour tillage, strip cropping on the contour, and terrace systems. Contour tillage is tillage operations and planting on the contour as contrasted to tillage up and down the slope or straight across the slope. No-till planting of corn on the contour between terraces in an Iowa field is shown in Figure 7.8. Limited field studies have shown that contour farming alone is effective in controlling erosion during rainstorms of low or moderate intensity, but it provides little protection against the occasional severe storm that causes breakovers by FIGURE 7.6 A continuous plant cover (e.g., runoff waters of the contoured ridges or rows. P bluegrass) is many times more effective for erosion values for contouring are given in Table 7.2, from control than cropping systems that leave the land which it is apparent that contouring is the most bare most of the time. Continuous corn production effective on slopes of 2.1 to 7 percent. Strip resulted in the longest amount of time with the soil unprotected by vegetation and the most erosion. cropping is the practice of growing two or more crops in alternating strips along contours, or per- covered with crop residues and 0.48 for continu- pendicular to surface water flow. Strip cropping, ous corn with conventional tillage. together with contour tillage, provides more protection. In cases where both strip cropping P = The Erosion Control Practice Factor and contour tillage are used, the P values listed in In general, whenever sloping land is cultivated Table 7.2 are divided by 2. and exposed to erosive rains, the protection of- Terraces are ditches that run perpendicular to

FIGURE 7.7 No-till corn with the residues of a previous soybean crop providing protection against soil erosion. (Photograph courtesy Soil Conservation Service, USDA.) 106 SOIL EROSION

FIGURE 7.8 Factors contributing to soil erosion control in this field include no-till tillage on the contour with terraces that greatly reduce the effective length of slope. (Photograph courtesy Soil Conservation Service, USDA.) the direction of surface water flow and collect ceptable levels of erosion. Having been devel- runoff water. The grade of the channel depends oped under the conditions found in the United on the amount of runoff water. In areas of limited States, one must be cautious in applying the equa- rainfall and permeable soil, channels may have a tion to drastically different conditions, such as zero grade. In humid regions, the terrace channel exist in the tropics. The procedure for computing usually is graded, and the runoff water that is the expected, average annual soil loss from a collected by the terrace is carried to an outlet for particular field is illustrated by the following ex- disposal. Terraces are an effective way to reduce ample. slope length. To account for terracing, the slope Assume that there is a field in Fountain County, length used to determine the LS factor should Indiana, on Russell silt loam. The slope has a represent the distance between terraces. Short gradient of 8 percent and is about 200 feet long. slope length between terraces is shown in Fig- The cropping system is a 4-year rotation of wheat- ure 7.8. alfalfa and bromegrass-corn-corn (W-AB-C-C) with tillage and planting on the contour, and with corn residues disked into the soil for wheat seed- Application of the Universal ing and turned under in the spring for second-year Soil-Loss Equation corn. Fertility and residue management on this The USLE was designed to predict the long-term farm are such that crop yields per acre are about erosion rates on agricultural land so that manage- 100 bushels of corn, 40 bushels of wheat, and 4 ment systems could be devised that result in ac- tons of alfalfa-bromegrass hay. The probability of an alfalfa-bromegrass failure is slight, and there is little risk that serious erosion will occur during the TABLE 7.2 Practice Factor Values for Contouring alfalfa-bromegrass year due to a lack of a good vegetative cover. The first step is to refer to the charts and tables discussed in the preceding sections and to select the values of R, K, LS, C, and P that apply to the specific conditions on this particular field. The value of the rainfall factor (R) is taken from Figure 7.4. Fountain County, in west-central In- From Wischmeier and Smith, 1965. diana, lies between isoerodents of 175 and 200. WATER EROSION 107

By linear interpolation, R equals 185. Soil scien- emphasis is on the maintenance of a certain thick- tists consider that the soil erodibility factor (K) for ness of soil overlying any impermable layer that Russell silt loam is similar to that for Fayette silt might exist. loam, being 0.38, according to Table 7.1. Second, the T value is the maximum average The slope-effect chart, Figure 7.5, shows that, annual soil loss that will allow continuous for an 8 percent slope, 200 feet long, the LS is 1.41. cropping and maintain soil productivity without For the productivity level and management prac- requiring additional management inputs. Fertil- tices, assumed in this example, the factor C for a izers are used to overcome the decline in soil W-AB-C-C rotation was shown earlier to be 0.119. fertility caused by erosion. Table 7.2 shows a practice factor of 0.6 for con- On thick, water-permeable soils, such as touring on an 8 percent slope. Marshall silt loam in western Iowa, artificial re- The next step is to substitute the selected nu- moval of the surface soil reduced yields to less merical values for the symbols in the soil erosion than one half of the crop yield obtained from equation, and solve for A. In this example: uneroded soil when no fertilizer was applied. The application of fertilizer containing 200 pounds of nitrogen per acre, however, resulted in similar yields of corn on both the normal soil and the soil If planting had been up and downslope instead with the surface soil removed. The Marshall soil is of on the contour, the P factor would have been a thick permeable soil that developed in a thick 1.0, and the predicted soil loss would have been layer of loess (parent material). Loess is a mate- 185 x 0.38 x 1.41 x 0.119 x 1.0 = 11.8 tons per rial transported and deposited by wind and con- acre (26.4 metric tons per hectare). sisting predominantly of silt-sized particles. Many If farming had been on the contour and com- soils that have developed from thick sediments of bined with minimum tillage for all corn in the loess are agriculturally productive. rotation, the factor C would have been 0.075. The There was some evidence that in years of mois- predicted soil loss would then have been 4.5 tons ture stress, the normal fertilized Marshall silt loam per acre (10.1 metric tons per hectare). outproduced the soil where the surface soil had been removed and fertilized. As a result of this and similar experiments, another approach was The Soil Loss Tolerance Value developed to establish Tvalues. This method con- The soil loss tolerance value (T) has been defined siders the maximum soil loss consistent with the in two ways. This is an indication of the lack of maintenance of productivity in terms of the main- concensus among soil scientists as to what ap- tenance of a rooting depth with desirable physi- proach should be taken with T values or how cal properties. Where subsoils have physical much erosion should be tolerated. First, the T properties unsuitable for rooting, erosion results value is the maximum soil erosion loss that is in reductions in soil productivity that cannot be offset by the theoretical maximum rate of soil overcome with only fertilizer application. The development, which will maintain an equilibrium data in Figure 7.9 show that removing the surface between soil losses and gains. Shallow soils over layer from a fine-textured Austin soil in the Black- hard bedrock have small T values. Erosion on lands of Texas resulted in longtime or permanent soils with claypans will result in shallower rooting reductions in soil productivity. Soil loss tolerance depths and in the need for smaller T values than value guidelines from the Soil Conservation Ser- for thick permeable soils, when both kinds of vice of the USDA are given in Table 7.3. soils have developed from thick and water- The data in Figure 7.6 show that average annual permeable unconsolidated parent materials. The soil loss with continuous bluegrass was 0.03 tons 108 SOIL EROSION

suited in a soil loss exceeding 50 tons per acre or 2.5 centimeters (1 inch) every 2 to 3 years, a loss far exceeding the Tvalue. In the Indiana case, the calculated soil losses ranged from about 4 to 12 tons per acre. The average annual rate of soil erosion on cropland land in the United States is 5 tons per acre. It has been estimated that soil erosion is the most important conservation problem on 51 percent of the nation's cropland. In general, Tvalues in the United States range from 1 to 5 tons per acre (2 to 11 metric tons per hectare). The USLE can be used to determine the management required to maintain soil losses at or below a certain T value.

Water Erosion on Urban Lands Historic soil erosion rates have changed as land use has changed. Much of the land in the eastern United States, which contributed much sediment FIGURE 7.9 Effect of removal of 38 centimeters of topsoil on grain sorghum yields on Austin clay at to streams and rivers a hundred or more years Temple, Texas, 1961-1965. (Data from Smith et al., ago, has been removed from cropping. Lower ero- 1967.) sion rates resulted from abandonment of land for cropping and returning it to forest. In the past few per acre and resulted in the removal of about 2.5 decades, much of this land has experienced a centimeters (1 in.) of soil every 5,000 years. Obvi- high erosion rate because of urbanization, con- ously, this soil loss is well below the T value for struction of shopping centers, roads, and housing the soil. On the other hand, continuous corn re- areas, as shown in Figure 7.10.

TABLE 7.3 Guide for Assigning Soil Loss Tolerance Values to Soils with Different Rooting Depths

Data from USDA-SCS, 1973. ° t Soils that have a favorable substratum and can be renewed by tillage, fertilizer, organic matter, and other management practices. $ Soils that have an unfavorable substratum, such as rock, and cannot be renewed economically. WATER EROSION 109

Now, about 50 percent of the sediment in where erosion occurs. Excessive water runoff and streams and waters throughout the country origi- soil erosion contribute to flooding and addition of nates on agricultural land; the other 50 percent fresh sediments to the soils of the north-China originates on urban lands. It has been noted that plain. exposed land on steep slopes is very susceptible Soil erosion results in the preferential loss of to erosion. In fact, exposure of land during high- the finer soil components that contribute the most way and building construction commonly results to soil fertility. Loss of organic matter, and the in erosion rates many times those that typically subsequent reduction in nitrogen mineralization, occur on agricultural land. Urban erosion rates as are particularly detrimental to the production of large as 350 tons per acre in a year have been nonleguminous grain crops. As mentioned, this reported. It is important to plan construction so loss can in some cases, be overcome by the addi- that land will be exposed a minmum period of tion of nitrogen fertilizer on responsive soils. time (see Figure 7.11). However, there is an increase in the cost of crop production. Exposure of clay-enriched Bt horizons (be- Water Erosion Costs cause of surface soil erosion) reduces water in- In northern China, there is an extensive loess de- filtration and increases water runoff. This results posit that is locally over 80 meters thick. This in more soil erosion and retention of less water for loess area is the major source of sediment for the plant growth. Seedbed preparation is more diffi- Huang Ho (Yellow) River. This sediment is the cult and the emergence of small plant seedlings major parent material for the alluvial soils of the may be reduced, resulting in a less than desirable vast north-China plain, which supports millions of density of plants. Summarization of data from people. The loessial landscape is very distinctive, many studies showed that 2.5 centimeters (1 in.) with deeply entrenched streams and gullies. It has of soil loss reduced wheat yield 5.3 percent, corn been estimated that the loess will be removed by 6.3 percent, and grain sorghum 5.7 percent. Some erosion in 40,000 years, which would be an an- effects of soil erosion on crop growth and produc- nual soil loss of 15 tons per acre (33 metric tons tion costs are given in Table 7.4. Increased proper hectare). This appears to be an excessive soil duction costs justify a long-term investment in erosion rate. erosion control measures to maintain erosion at The costs and benefits of the erosion and sedi- or below acceptable T values. mentation are difficult to assess. In the short term, Off-site costs result from sediment in water set- food production costs are increased in areas tling and filling reservoirs and navigation chan- 11 0 SOIL EROSION

FIGURE 7.11 Slurry truck spraying fertilizer and seed on freshly prepared seedbed along road in Missouri. A thin layer of straw mulch will be secured with a thin spray of tar to protect the soil until vegetation becomes established. (Photograph courtesy Soil Conservation Service, USDA.)

nels. The sediments adversely affect the ecology in diameter) are rolled over the surface by direct of streams and lakes, as well as municipal water wind pressure, then suddenly jump up almost supplies. In 1985 it was estimated that the cost of vertically over a short distance to a height of 20 to soil erosion resulting from all phases of pollution, 30 centimeters. Once in the air, particles gain in both agricultural and urban, was $6 billion annu- velocity and then descend in an almost straight ally in the United States. This is a loss greater than line, not varying more than 5 to 12 degrees from that estimated for the loss in soil productivity from the horizontal. The horizontal distance traveled by the eroded land. a particle is four to five times the height of its jump. On striking the surface, the particles may rebound into the air or knock other particles into WIND EROSION the air before coming to rest. The major part of soil carried by wind moves by this process. About Wind erosion is indirectly related to water conser- 93 percent of the total soil movement via wind vation in that a lack of water leaves land barren takes place below a height of 30 centimeters. and exposed to the wind. Wind erosion is greatest Probably 50 percent or more of movement occurs in semiarid and arid regions. In North Dakota, between 0 and 5 centimeters above the soil wind-erosion damage on agricultural land was surface. reported as early as 1888. In Oklahoma, soil ero- Very fine dust particles are protected from wind sion was reported 4 years after breaking of the action, because they are too small to protrude sod. Blowing dust was a serious hazard of early above a minute viscous layer of air that clings to settlers on the Plains, and was one of the first the soil surface. As a result, a soil composed of problems studied by the Agricultural Experiment extremely fine particles is resistant to wind ero- Stations of the Great Plains. sion. These dust-sized particles are thrown into the air chiefly by the impact of particles moving in saltation. Once in the air, however, their move- Types of Wind Erosion ment is governed by wind action. They may be Soil particles move in three ways during wind carried very high and over long distances via sus- erosion: saltation, suspension, and surface creep. pension. During saltation, fine soil particles (0.1 to 0.5 mm Relatively large sand particles (between 0.5 and WIND EROSION 11 1

TABLE 7.4 Effects of Erosion on Corn Growth and Production Costs

1.0 mm in diameter) are too heavy to be lifted by field length, and V equals the quantity of vegeta- wind action, but they are rolled or pushed along tive cover. Wind erosion tolerance levels (T the surface by the impact of particles during salta- values) have been established, just as for water tion. Their movement is by surface creep. erosion. The use of the wind erosion equation Between 50 and 75 percent of the soil is carried is more complex than that used for calculating by saltation, 3 to 40 percent by suspension, and 5 water erosion losses. Details for use of the to 25 percent by surface creep. From these facts, it wind erosion equation are given in the National is evident that wind erosion is due principally to Agronomy Manual of the Soil Conservation Ser- the effect of wind on particles of a size suitable to vice, 1988. move in saltation. Accordingly, wind erosion can be controlled if: (1) soil particles can be built up into peds or granules too large in size to move by Factors Affecting Wind Erosion saltation; (2) the wind velocity near the soil sur- Soil erodibility by wind is related primarily to soil face is reduced by ridging the land, by vegetative texture and structure. As the clay content of soils cover, or even by developing a cloddy surface; increases, increased aggregation creates clods or and (3) strips of stubble or other vegetative cover peds too large to be transported. A study in west- are sufficient to catch and hold particles moving ern Texas showed that soils with 10 percent clay in saltation. eroded 30 to 40 times faster than soil with 25 percent clay, as shown in Figure 7.12. Rough soil surfaces reduce wind erosion by Wind Erosion Equation reducing the surface wind velocity and by trap- The factors that affect wind erosion are contained ping soil particles. Surface roughness can be in- in the equation to estimate soil loss by wind creased with tillage operations, as shown in Fig- erosion: ure 7.13. Tillage and rows are positioned at right angles to the dominant wind direction for maximum effectiveness. where: E equals the soil loss in tons per acre, I Climate is a factor in wind erosion via its effect equals the soil erodibility, K equals the soil rough- on wind frequency and the velocity and wetness ness, C equals the climatic factor, L equals the of soil during high-wind periods. Wind erosion 11 2 SOIL EROSION

and blowing dust are persistent features of deserts in which plants are widely spaced and dry soil is exposed most of time. When surface soils contain a wide range in particle sizes, wind erosion preferentially removes the finer particles, leaving the larger particles behind. In this way a desert pavement is created, consisting of gravel particles that are too large and heavy to be blown away. Consequently, deserts are an important source of loess, which is composed mainly of silt-sized particles. Furthermore, climate affects the amount of vegetative cover. During the Dust Bowl of the 1930s, both wheat yields and wind erosion were related to rainfall. The lowest yields and highest soil erosion rates occurred during years of least rainfall. Wind erosion increases from zero, at the edge of an open field, to a maximum, with increasing distance of soil exposed to the wind. As particles FIGURE 7.12 Amount of wind erosion in relation to bounce and skip, they create an avalanche effect the amount of clay in the soil. (Data from Chepil et on soil movement. Windbreaks of trees and al- al., 1955.) ternate strips of crops can be used to reduce the

FIGURE 7.13 Contour ridges reduce wind velocity and collect much of the blowing soil. (Photograph courtesy soil Conservation Service, USDA.) REFERENCES 11 3 effective field length exposed to the wind. Finally, with a heavy roller induces moisture to rise more wind erosion is inversely related to the degree of rapidly by capillarity, thus dampening the soil vegetative cover. Growing crops and the residues surface. The water table must be quite close to the of previous crops are effective in controlling wind soil surface for this practice to be effective. erosion. Stubble mulch tillage-tillage that leaves plant residues on the soil surface-is popular in the wheat producing region of the Great Plains. SUMMARY The factors that affect soil erosion by water are Deep Plowing for Wind Erosion Control rainfall, soil erodibility, slope length and gradient, Deep plowing has been used to control wind ero- crop management practices, and erosion control sion where sandy surface soils are underlain by Bt practices. The universal soil loss equation is used horizons that contain 20 to 40 percent clay. In to estimate soil erosion loss on agricultural land. Kansas and Texas, plowing to control wind ero- The permissible soil loss is the T value. The sion must be deep enough to include at least I universal soil loss equation can be used to deter- centimeter of Bt horizon for every 2 centimeters of mine the soil management systems that will result the surface soil (A horizon) thickness. In many in acceptable erosion levels. places, the soil must be plowed to a depth of 50 to Soil erosion is greatly accelerated when the 60 centimeters. This kind of plowing increases the vegetative cover is removed. Thus, soil erosion is clay content of the surface soil by 5 to 12 percent, a serious problem in both rural and urban areas. in some cases. Since 27 percent clay in the sur- Cost of soil erosion includes reduced soil pro- face was required to halt soil blowing, deep plow- ductivity and higher crop production costs. Offsite ing by itself resulted in only partial and temporary costs are due to silting of watercourses and reser- control of wind erosion. When deep plowing was voirs and reduced water quality. not accompanied by other soil erosion control Wind erosion is a major problem when sandy practices, wind erosion removed clay from the soils are used for crop production in regions with surface soil, and the effects of deep plowing were dry seasons, where the soil surface tends to be temporary. exposed to the wind because of limited vegetative cover. Wind erosion control is based on one or Wind Erosion Control on Organic Soils more of the following: Organic soil areas of appreciable size are frequently protected from wind erosion by planting 1. Trapping soil particles with rough surface trees in rows-windbreaks. The use of trees as and/or the use of crop residues and strip windbreaks must be limited, however, because of cropping. the large amount of soil that they take out of crop 2. Deep plowing to increase clay content of sur- production. A number of shrubs are also used; for face soil and simultaneous use of other ero- example, spirea is popular as a windbreak. Trees sion control practices. and shrubs also provide cover for wildlife. 3. Protecting the soil surface with a vegetative The lightness of dry organic soils contributes to cover. the wind erosion hazard. Moist organic soil is heavier than dry organic soils, and particles tend to adhere more, reducing the wind erosion haz- REFERENCES ard. Accordingly, use of an overhead sprinkler Anonymous. 1982. Soil Erosion: Its Agricultural, Envi- irrigation system is helpful during windy periods, ronmental, and Socioeconomic Implications. Coun- before the crop cover is sufficient to provide cil for Agr. Sci. and Technology Report No. 92, Ames, protection from the wind. Rolling the muck soil Iowa. 11 4 SOIL EROSION

Beasley, R. P. 1974. "How Much Does Erosion Cost?" and D. 0. Thompson. 1967. "Renewals of Desurfaced Soil Survey Horizons. 15:8-9. Austin Clay." Soil Sci. 103:126-130. Browning, G. M., R. A. Norton. A. G. McCall, and F. G. Soil Conservation Service, 1988. National Agronomy Bell. 1948. "Investigation in Erosion Control and the Manual. USDA, Washington, D.C. Reclamation of Eroded Land at the Missouri Valley Tuan, Yi-Fu. 1969. China. Aldine, Chicago. Loess Conservation Experiment Station." USDA Tech USDA-SCS. 1973. Advisory Notice, 6. Washington, D.C. Bull. 959, Washington, D.C. Wischmeier, W. H. and D. D. Smith. "Predicting Burnett, E., B. A. Stewart, and A. L. Black. 1985. "Re- Rainfall-Erosion Losses from Cropping East of the gional Effects of Soil Erosion on Crop Productivity- Rocky Mountains." USDA Agr. Handbook 282. Wash- Great Plains." Soil Erosion and Crop Productivity. ington, D.C. American Society of Agronomy, Madison, Wis. Wischmeier, W. H. and D. D. Smith. 1978. "Predicting Chepil, W. S., N. P. Woodruff, and A. W. Zingg. 1955. Rainfall Erosion Loss-A Guide to Conservation Plan- "Field Study of Wind Erosion in Western Texas." ning." USDA Agr. Handbook 537. Washington, D.C. Kansas and Texas Agr. Exp. Sta. and USDA Washing- Woodruff, N. P. and F. H. Siddoway. 1965. "A Wind ton, D.C. Erosion Equation." Soil Sci. Soc. Am. Proc. 29:602- Colacicco, D., T. Osborn, and K. Alt. 1989. "Economic 608. Damage from Soil Erosion." J. Soil and Water Con. Wolman, M. G. 1967. "A Cycle of Sedimentation and 44:(1), 35-39. Erosion in the Urban River Channels." Geog. Ann. Massey, H. F. and M. L. Jackson. 1952. "Selective Ero- 49A:385-395. sion of Soil Fertility Constitutents." Soil Sci. Soc. Am. Zobeck, T. M., D. W. Fryrear, and R. D. Petit. 1989. Proc. 16:353-356. "Management Effects on Wind-eroded Sediment and Smith, R. M., R. C. Henderson, E. D. Cook, J. E. Adams, Plant Nutrients." J Soil Water Con. 44:160-163. Im CHAPTER 8

SOIL ECOLOGY

The soil is the home of innumerable forms of THE ECOSYSTEM plant, animal, and microbial life. Some of the The sum total of life on earth, together with the fascination and mystery of this underworld has global environments, constitutes the ecosphere. been described by Peter Farb: The ecosphere, in turn, is composed of numerous We live on the rooftops of a hidden world. Be- self-sustaining communities of organisms and neath the soil surface lies a land of fascination, their inorganic environments and resources, and also of mysteries, for much of mans wonder called ecosystems. Each ecosystem has its own about life itself has been connected with the soil. unique combination of living organisms and It is populated by strange creatures who have abiotic resources that function to maintain a con- found ways to survive in a world without sunlight, tinuous flow of energy and nutrients. an empire whose boundaries are fixed by earthen All ecosystems have two types of organisms walls. based on carbon source: (1) producers, and (2) the consumers and decomposers. The producers Life in the soil is amazingly diverse, ranging use (fix) inorganic carbon from carbon dioxide, from microscopic single-celled organisms to and are autotrophs. The consumers and the de- large burrowing animals. As is true with organ- composers use the carbon fixed by the producers, isms above the ground, there are well-defined such as glucose, and are heterotrophs. food chains and competition for survival. The study of the relationships of these organisms in the soil environment is called soil ecology. Producers The major primary producers are vascular plants that use solar energy to fix carbon from carbon

* Peter Farb, Living Earth, Harper and Brothers Publishers, dioxide during photosynthesis. The tops of plants New York, 1959. provide food for animals above the soil-

115 11 6 SOIL ECOLOGY

atmosphere interface. Plants produce roots, tu- of an element from an organic form to an inor- bers, and other underground organs within the ganic state as a result of microbial activity. An soil that serve as food for soil-dwelling organisms. example is the conversion of nitrogen in protein A very small amount of carbon is fixed from car- into nitrogen in ammonia (NH3). The microor- bon dioxide by algae during photosynthesis that ganisms are considered to be the major or ulti- occurs at or near the soil surface. Some bacteria mate decomposers. They are the most closely obtain their energy from chemical reactions, che- associated with the ultimate release of the energy moautotrophs, and fix a tiny amount of carbon and nutrients in decomposing materials and in from carbon dioxide. The material produced by making the nutrients available for another cycle of the producers serves as food for the consumers plant growth. Only about 5 percent of the primary and decomposers. production of green plants is consumed by animals. By contrast, about 95 percent of the primary production is ultimately decomposed by mi- Consumers and Decomposers croorganisms. Consumers are, typically, animals that feed on plant material or on other animals. For example, very small worms invade and eat living roots. The General Features of Decomposers worms might be eaten or consumed by mites The decomposers secrete enzymes that digest or- which, in turn might be consumed by centipedes. ganic matter outside the cell, and they absorb the Eventually, all organisms die and are added to the soluble end products of digestion. Different en- soil, together with the waste material of animals. zymes are excreted, depending on the decom- All forms of dead organic materials are attacked posing material. For example, the enzyme cellu- by the decomposers, mainly by bacteria and lase is excreted to decompose cellulose and the fungi. Through enzymatic digestion (decom- enzyme protease is excreted to decompose position), the carbon is returned to the atmo- protein. These enzymes and processes are similar sphere as C0 2 and energy is released as heat. The to those that occur in the digestive system of nutrients in organic matter are mineralized and animals. appear as the original ions that were previously The microorganisms near plant roots share the absorbed by plant roots. The result is that the same environment with roots, and they compete major function of the consumers and decom- with each other for the available growth factors. posers is the cycling of nutrients and energy. In When temperature, nutrients and water supply, this context it is appropriate to consider soil as the pH, and other factors are favorable for plant stomach of the earth. Without decomposers to growth, the environment is generally favorable for release fixed carbon, the atmosphere would be- the growth of microorganisms. Both absorb nutri- come depleted of C0 2 , life would cease, and the ent ions and water from the same soil solution. cycle would stop. Both are affected by the same water potentials, osmotic effects of salts, pH, and gases in the soil atmosphere. Both may be attacked by the same predators. On the other hand, plants and microor- MICROBIAL DECOMPOSERS ganisms also play roles that benefit each other. All living heterotrophs, animals and microor- Roots slough off cells and leak organic com- ganisms, are both consumers and decomposers. pounds that serve as food for microorganisms, They are consumers in the sense that they con- whereas the microbes decompose the organic sume materials for growth and, at the same time, materials, which results in the mineralization of they are decomposers in the sense that carbon is nutrient ions for root absorption. released as C02 . Mineralization is the conversion In the case of symbiotic nitrogen fixation, the MICROBIAL DECOMPOSERS 11 7 nitrogen-fixing organisms obtain energy and food from the plant while the plant benefits from the nitrogen fixed. Nitrogen fixation is the conversion of molecular nitrogen (N 2) to ammonia and subsequently to organic combinations or to forms utilizable in biological processes. The net effect of the fixation is the transfer of nitrogen in the atmosphere, which is unavailable to plants, to plants and soils where the nitrogen is available. The major distinguishing feature of microorganisms is their relatively simple biological organization. Many are unicellular, and even multi- cellular organisms lack differentiation into cell types and tissues characteristic of plants and animals. They form spores, or a resting stage, under unfavorable conditions and germinate and grow again when conditions become favorable.

Bacteria Bacteria are single celled, among the smallest FIGURE 8.1 Rod-shaped bacteria magnified 20,000 living organisms, and exceed all other soil organ- times. (Courtesy Dr. S. Flegler of Michigan State isms in kinds and numbers. A gram of fertile soil University). commonly contains 10' to 10 10 bacteria. The most common soil bacteria are rod-shaped, a micron (1 /25,000 of an inch) or less in diameter, and up ply very rapidly under favorable conditions. If a to a few microns long (see Figure 8.1). Research- single bacterium divided every hour, and every ers have estimated that the live weight of bacteria subsequent bacterium did the same, 17 million in soils may exceed 2,000 kilograms per hectare cells would be produced in a day. Such a rapid (2,000 pounds per acre). The estimated weights growth rate cannot be maintained for a long pe- and numbers of some soil organisms are given in riod of time because nutrients and other growth Table 8.1. factors are exhausted and waste products accu- Most of the soil bacteria are heterotrophs and mulate and become toxic. In general, the growth require preformed carbon (such as carbon in or- of the decomposers is chronically limited by the ganic compounds). Most of the bacteria are small amount of readily decomposable substrate aerobes and require a supply of oxygen (0 2) in the that is available as a food source. Most of the soil atmosphere. Some bacteria are anaerobic organic matter in soils is humus; organic material and can thrive only with an absence of oxygen in that is very resistant to further enzymatic decom- the soil atmosphere. Some bacteria, however, are position. facultative aerobes. They thrive well in an aerobic environment but they can adapt to an anaerobic environment. Fungi Bacteria normally reproduce by binary fission Fungi are heterotrophs that vary greatly in size and (the splitting of a body into two parts). Some structure. Fungi typically grow or germinate from divide as often as every 20 minutes and may multi- spores and form a threadlike structure, called the 11 8 SOIL ECOLOGY mycelium. Whereas the activity of bacteria is lim- Some molds grow on plant leaves and produce ited to surface erosion in place, fungi readily ex- white cotton-colored colonies that produce a dis- tend their tissue and penetrate into the surround- ease called downy mildew. Mushroom fungi have ing environment. The mycelium is the working an underground mycelium that absorbs nutrients structure that absorbs nutrients, continues to and water, and an above-ground mushroom that grow, and eventually produces reproductive contains reproductive spores. Many mushrooms structures that contain spores, as shown in Fig- are collected for food, such as the shaggy-mane ure 8.2. mushroom shown in Figure 8.3. It is difficult to make an accurate determination Fungi are important in all soils. Their tolerance of the number of fungi per gram of soil, because for acidity makes them particularly important in mycelia are easily fragmented. Mycelial threads or acidic forest soils. The woody tissues of the forest fragments have an average diameter of about 5 floor provide an abundance of food for certain microns, which is about 5 to 10 times that of fungi that are effective decomposers of lignin. In bacteria. A gram of soil commonly contains 10 to the Sierra Nevada Mountains, there is a large 100 meters of mycelial fragments per gram. On mushroom fungus, called the train wrecker fun- this basis, the live weight of fungal tissue in soils gus, because it grows abundantly on railroad ties is about equal to that of bacteria (see Table 8.1). unless the ties are treated with creosote. The most common fungi are molds and mushrooms. Mold mycelia are commonly seen growing on bread, clothing, or leather goods. Rhizopus is Actinomycetes a common mold that grows on bread and in soil. It Actinomycetes refers to a group of bacteria with a has rootlike structures that penetrate the bread superficial resemblance to fungi. The actino- and absorb nutrients, much like the roots of mycetes resemble bacteria in that they have a very plants. The structure on the surface of the bread simple cell structure and are about the same size forms the working structure, or mycelium, from in cross section. They resemble filamentous fungi which fruiting bodies that contain spores develop. in that they produce a branched filamentous network. The network compared to fungi, however, is FIGURE 8.2 A soil fungus showing mycelium and usually less extensive. Many of these organisms reproductive structures that contain spores. reproduce by spores, which appear to be very (Photograph courtesy of Michigan State University much like bacterial cells. Pesticide Research Electron Microscope Laboratory.) Actinomycetes are in great abundance in soils, as shown in Table 8.1. They make up as much as 50 percent of the colonies that develop on plates containing artificial media and inoculated with a soil extract. The numbers of actinomycetes may vary from 1 to 36 million per gram of soil. Al- though there is evidence that actinomycetes are abundant in soils, it is generally concluded they that are not as important as bacteria and fungi as decomposers. It appears that actinomycetes are much less competitive than the bacteria and fungi when fresh additions of organic matter are added to soils. Only when very resistant materials remain do actinomycetes have good competitive ability. SOIL ANIMALS 11 9

TABLE 8.1 Estimates of Amount of Organic Matter and Proportions, Dry Weight, and Number of Living Organisms in a Hectare of Soil to a Depth of 15 Centimeters in a Humid Temperate Region

Vertical Distribution of Decomposers in SOIL ANIMALS the Soil The surface of the soil is the interface between the Soil animals are numerous in soils (see Table lithosphere and the atmosphere. At or near this 7.1). Soil animals can be considered both con- interface, the quantity of living matter is greater sumers and decomposers because they feed on or than at any region above or below. As a conse- consume organic matter and some decom- quence, the A horizon contains more organic position occurs in the digestive tract. Animals, debris or food sources than do the B and C hori- however, play a minor consumer-decomposer zons. Although other factors besides food supply role in organic matter decomposition. Some influence activity and numbers of microor- animals are parasitic vegetarians that feed on ganisms, the greatest abundance of decomposers roots, whereas others are carnivores that prey on typically occurs in the A horizon (see Figure 8.4). each other. 120 SOIL ECOLOGY

into United States from Europe. This worm makes a shallow burrow and forages on plant material at night. Some of the plant material is dragged into the burrow. In the soil, the plant materials are moistened and more readily eaten. Other kinds of earthworms exist by ingesting organic matter found in the soil. Earthworms eat their way through the soil by ingesting the soil en masse. Excreted materials (earthworm castings) are deposited both on and within the soil. Although castings left on the soil surface of lawns are objec- tionable, earthworms feed on thatch, thus' helping to prevent thatch buildup in turf grass. The intimate mixing of soil materials in the digestive track of earthworms, the creation of channels, and production of castings, alter soil structure and leave FIGURE 8.3 Mushroom fungal caps that contain spores-an edible type. the soil more porous. Channels left open at the soil surface greatly increase water infiltration. Earthworms prefer a moist environment with an abundance of organic matter and a plentiful sup- Worms ply of available calcium. As a result, they tend to There are two important kinds of worms in soils. be least abundant in dry and acidic sandy soils. Microscopic roundworms, nematodes, are very They normally avoid water-saturated soil. If they abundant soil animals. They are of economic im- emerge during the day, when it is raining, they are portance because they are parasites that invade killed by ultraviolet radiation unless they quickly living roots. The other important worm is the ordi- find protection. Obviously, the number and activ- nary earthworm. ity of earthworms vary greatly from one location to another. Suggestive numbers of earthworms in an Ap horizon vary from a few hundred to more than Earthworms Earthworms are perhaps the best a million per acre. Estimated weights of earth- known of the larger soil animals. The common worms are 100 to 1,000 pounds per acre (kilo- earthworm, Lumbricus terrestris, was imported grams per hectare). Attempts to increase earth-

FIGURE 8.4 Distribution of microorganisms in the A, B, and C horizons of a cultivated grassland soil. All values refer to the number of organisms per gram of air-dry soil. (Data from Iowa Res. Bul. 132.) SOIL ANIMALS 121 worm activities in soils have been disappointing, the case of a root vegetable such as carrots, the because the numbers present probably represent market quality is seriously affected. an equilibrium population for the existing condi- Investigations indicate that nematode damage tions. is much more extensive than previously thought. Nematodes are a very serious problem for pineapple production in Hawaii, where the soil for a new Nematodes Nematodes (roundworms) are mi- planting is routinely fumigated to reduce nema- croscopic worms and are the most abundant tode populations. Parasititic nematodes attack a animals in soils. They are round shaped with a wide variety of plants throughout the world, re- pointed posterior. Under a 10-power hand lens ducing the efficiency of plant growth, analogous they appear as tiny transparent threadlike worms, to worms living in the digestive track of animals. as illustrated in Figure 8.5. Most nematodes are Agriculturally, the most important role of nema- free-living and inhabit the water films that sur- todes in soils appears to be economic, as para- round soil particles. They lose water readily sitic consumers. through their skin, and when soils dry or other unfavorable conditions occur, they encyst, or form a resting stage. Later, they reactivate when Arthropods conditions become favorable. Based on food pref- A high proportion of soil animals is arthropods; erences, nematodes can be grouped into those they have an exoskeleton and jointed legs. Most that feed on: (1) dead and decaying organic mat- have a kind of heart and blood system, and usu- ter, (2) living roots, and (3) other living organisms ally a developed nervous system. The most abun- as predators. dant arthropods are mites and springtails. Other Parasitic nematodes deserve attention owing to important soil arthropods include spiders, insects their economic importance in agriculture. Many (including larvae), millipedes, centipedes, wood plants are attacked, including tomatoes, peas, lice, snails, and slugs. carrots, alfalfa, turf grass, ornamentals, corn, soybeans, and fruit trees. Some parasitic nematodes have a needlelike anterior end (stylet) used Springtails Springtails are primitive insects to pierce plant cells and suck out the contents. less than 1 millimeter (2s in.) long. They are dis- Host plants respond in numerous ways: plants, tributed worldwide and are very abundant (see for example, develop galls or deformed roots. In Table 8.1). Springtails have a springlike ap-

FIGURE 8.5 A nematode and a piece of ordinary cotton thread photographed at the same magnification (270). The nematode is ,'55 as thick as the thread and is not visible to the naked eye. (Courtesy H. H. Lyon, Plant Pathology Department, Cornell University.) 122 SOIL ECOLOGY pendage under their posterior end that permits them to flit or spring in every direction. If the soil in which they are abundant is disturbed and broken apart, they appear as small white dots darting to and fro. They live in the macropores of the litter layers and feed largely on dead plant and animal tissue, feces (dung), humus, and fungal mycelia. Water is lost rapidly through the skin, so they are restricted to moist layers. Springtails in the lower litter layers are the most primitive and are without eyes and pigment. Springtails are in the order Collembola, and FIGURE 8.7 Electron microscope photograph of a they are commonly called Collembola. Their ene- mite on the top of a pinhead; magnified about 50 to mies include mites, small beetles, centipedes, 100 times. (Photograph courtesy Michigan State and small spiders. Collembola that were grown in University Pesticide Research Laboratory.) the laboratory, and are in various stages of development, are shown in Figure 8.6. todes, insect eggs, and other small animals, including springtails. Activities of mites include the Mites Mites are the most abundant air- breakup and decomposition of organic material, breathing soil animals. They commonly have a movement of organic matter to deeper soil layers, saclike body with protruding appendages and are and maintenance of pore spaces (runways). related to spiders. They are small, like Collem- bola; a mite photographed on the top of a pinhead is shown in Figure 8.7. Millipedes and Centipedes Millipedes and Some mites are vegetarians and some are carni- centipedes are elongate, fairly large soil animals, vores. Most feed on dead organic debris of all with many pairs of legs. They are common in kinds. Some are predaceous and feed on nema- forests, and overturning almost any log or stone will send them running for cover. Millipedes have many pairs of legs and are mainly vegetarians. They feed mostly on dead organic matter, but FIGURE 8.6 Springtails and eggs photographed through a light microscope. some browse on fungal mycelia. Centipedes typically have fewer pairs of legs than millipedes, and are mainly carnivorous consumers. Centipedes will attack and consume almost any-sized animal that they can master (see Figure 8.8). Worms are a favorite food of some centipedes. The data in Table 8.1 show that the numbers of millipedes and centipedes are small compared with springtails and mites, but that their biomass may be larger.

White Grubs Some kinds of insect larvae playa more important role in soils than do the adults as NUTRIENT CYCLING 123

animals. The excrement of animals is attacked by microbes and ingested by animals. Thus, material is subjected to decomposition in several stages and in widely different environments. Organic matter, and the soil en masse, may be ingested by earthworms, thereby producing an intimate mixing of organic and mineral matter. Again, digestion results in some decomposition and, ultimately, the material is excreted as castings. The net result of these activities is the mineralization of elements in organic matter, conversion of carbon to C0 2 , and release of energy as heat. The FIGURE 8.8 A centipede, a common carnivorous major credit for nutrient mineralization and recyl- soil animal. ing of nutrients and energy, however, goes to the microbes (bacteria and fungi). does, for example, the white grub. White grubs are larvae of the familar May beetle or june bug. The grubs are round, white, about 2 to 3 cen- NUTRIENT CYCLING timeters long, and curl into a C shape when disturbed. The head is black and there are three pairs Nutrient cycling is the exchange of nutrient ele- of legs just behind the head. White grubs feed ments between the living and nonliving parts of mainly on grass roots and cause dead spots in the ecosystem. Plants and microbes absorb nutri- lawns. A wide variety of other plants are also ents and incorporate them into organic matter, attacked, making white grubs an important agri- and the microbes (with aid of animals) digest the cultural pest. Moles feed on insect larvae and organic matter and release the nutrients in min- earthworms, resulting in a greater likelihood of eral form. Nutrient cycling conserves the nutrient mole damage in lawns when white grubs are supply and results in repeated use of the nutrients present. in an ecosystem. Ants and termites are important soil insects and will be considered later in relation to the earth- moving activities of soil animals. Nutrient Cycling Processes Two simultaneous processes, mineralization and immobilization, are involved in nutrient cycling. Interdependence of Microbes and Immobilization is the uptake of inorganic ele- Animals in Decomposition ments (nutrients) from the soil by organisms and Microorganisms and the animals work together as conversion of the elements into microbial and a decomposing team. When a leaf falls onto the plant tissues. These nutrients are used for growth forest floor, both micoorganisms and animals at- and are incorporated into organic matter. Mineral- tack the leaf. Holes made in the leaf by mites and ization is the conversion of the elements in or- springtails facilitate the entrance of microor- ganic matter into mineral or ionic forms such as Cat-, - 2- ganisms inside the leaf. Soil animals also break NH 3 , H2P0 4 , S0 4 , and K+. These ions up organic debris and increase the surface area then exist in the soil solution and are available for for subsequent attack by bacteria and fungi. Soil another cycle of immobilization and mineral- animals ingest bacteria and fungi, which continue ization. to function within the digestive tracks of the Mineralization is a relatively inefficient process 124 SOIL ECOLOGY

in that much of the carbon is lost as CO 2 and much of the energy escapes as heat. This typically produces a supply of nutrients that exceeds the needs of the decomposers; the excess of nutrients released can be absorbed by plant roots. Only in unusual circumstances is there serious competition between the plants and microbes for nutrients.

A Case Study of Nutrient Cycling One approach to the study of nutrient cycling is to select a small watershed and to measure changes in the amounts and locations of the nutrients over time. An example is a 38-acre (15-hectare) watershed in the White Mountains of New Hampshire. In this mature forest, the amount of nutrients taken up from the soil approximated the amount of nutrients returned to the soil. For calcium, this was 49 kilograms per hectare (44 lb per acre) taken up from the soil and returned to the soil, as illustrated in Figure 8.9. About 9 kilograms of calcium per hectare were added to the system by mineral weathering and 3 kilograms per hectare were added in the precipitation. These additions FIGURE 8.9 Calcium cycling in a hardwood forest were balanced by losses due to leaching and run- in New Hampshire in kilograms per hectare per year. off. This means, in effect, that 80 percent of the (Data from Bormann and Likens, 1970.) calcium in the cycle was recycled or reused by the forest each year. Plants were able to take up 49 kilograms of calcium per hectare, whereas only Effect of Crop Harvesting on 12 kilograms were added to the system annually. Nutrient Cycling Researchers concluded that northern hardwood The grain and stalks (stover) of a corn (Zea forests have a remarkable ability to hold and to maize) crop may contain nitrogen equal to more recirculate nutrients. than 200 kilograms per hectare (200 lb per acre). It The data in Table 8.2 show that 70 to 86 percent is not uncommon for farmers in the Corn Belt of four major nutrients absorbed from the soil region of the United States to add this amount of were recycled by return of the nutrients in leaves nitrogen to the soil annually to maintain high and wood to the litter layer each year. Without this yields. According to the data in Table 8.2, by con- extensive recycling, the productivity of the forests trast, only 10 kilograms of nitrogen per hectare would be much lower. Nutrient cycling accounts were stored annually in wood or removed from for the existence of enormous forests on some the system by tree growth in a beech forest. Natu- very infertile soils. In some very infertile soils, rally occurring nitrogen fixation and the addition most of the nutrients exist in the organic matter via precipitation can readily supply this amount of and are efficiently recycled. nitrogen. Basically, food production represents SOIL MICROBE AND ORGANISM INTERACTIONS 12 5

TABLE 8.2 Annual Uptake, Retention, and Return of Nutrients to the Soil in a Beech Forest

Stoeckler, J. H., and H. F. Arneman, "Fertilizers in Forestry," Adv. in Agron. 12:127-195, 1960. By permission of author and Academic Press. nutrient harvesting. The natural nutrient cycling tion of manure to the land. Waste disposal is an processes cannot provide enough nutrients to important problem. Conversely, large crop farms grow high-yielding crops. Yields of grain in many without animals have no manure to apply to the of the world's agricultural systems stabilize at land, so they balance the nutrient cycle deficit only 500 to 600 kilograms per hectare (8 to 10 with the application of fertilizer. bushels per acre) where there is little input of nutrients as manure or fertilizer. When the agricultural system is based on feed- SOIL MICROBE AND ing most of the crop production to animals, as in ORGANISM INTERACTIONS dairying, most of the nutrients in the feed are excreted by the animals as manure. On the aver- The role of soil organisms in nutrient cycling has age, 75 to 80 percent of the nitrogen, 80 percent of been stressed, and it has been shown that some the phosphorus, and 85 to 90 percent of the po- organisms are parasites that feed on plant roots. tassium in the feed that is eaten by farm animals is In this section the specialized roles of microor- excreted as manure. Crops can be harvested with ganisms living near and adjacent to plant roots only a modest drain on the nutrients in the cycle if will be considered. the nutrients in the manure are returned to the land. Careful management of manure and the use of legumes to fix nitrogen, together with lime to The Rhizosphere reduce soil acidity, were the most important prac- Plant roots leak or exude a large number of or- tices used to develop a prosperous agriculture in ganic substances into the soil. Sloughing of root dairy states like New York and Wisconsin. caps, and other root cells, provide much new Specialization in agriculture has resulted in organic matter. More than 25 percent of the photo- very large crop and animal farms. Today, many synthate produced by plants may be lost from the large livestock enterprises have more manure, roots to the soil. These substances are food for and nutrients, than they can dispose of by applica- microorganisms, and they create a zone of in- 126 SOIL ECOLOGY tense biological activity near the roots in an area ganisms quickly die when added to soils. Some called the rhizosphere. The rhizosphere is the human disease organisms, however, are very per- zone of soil immediately adjacent to plant roots in sistent. The disposal of human feces by back- which the kinds, numbers, or activities of micro- packers in wilderness areas poses a health threat, organisms differ greatly from that of the bulk soil. because some organisms are not readily killed by Although many kinds of organisms inhabit the shallow burial. There is also danger of causing rhizosphere, it appears that bacteria are benefited disease in humans when human feces are used the most. The bacteria appear mainly in colonies for fertilizing gardens or fields, because of resis- and may cover 4 to 10 percent of the root surface. tant spore forms of some disease organisms. In The intimate association of bacteria and roots China, much of the human excrement is used for may result in mutually beneficial or detrimental fertilizer, but this material is composted for a suf- effects. Nutrient availability may be affected and ficiently long period of time to kill the disease plant-growth-stimulating or toxic substances may organisms before the compost is applied to the be produced. Since roots tend to grow better in fields. sterile soil inoculated with organisms than in ster- Historically, human excrement (night soil) has ile soil that is not inoculated, it appears that the played an important role in the maintenance net effect of bacteria growing in the rhizosphere is of soil fertility. In 1974 the value of the nitro- beneficial. gen, phosphorus, and potassium in the human excreta in India was estimated to be worth more than $700 million as compared with the cost of the Disease nutrients as fertilizer. In countries where most of Soils may contain organisms that cause both the food is consumed by humans, careful use of plant and animal disease. Bacteria are responsi- human excrement can be very helpful in main- ble for wilt of tomatoes and potatoes, soft rots of a taining soil fertility. In highly industrialized soci- number of vegetables, leaf spots, and galls. Some eties, the nutrients are disposed of through sew- fungi cause damping-off of seedlings, cabbage age systems and lost from the food production yellows, mildews, blight, and certain rusts. The enterprise. The low cost of energy means that the catastrophic potato famine in Ireland from 1845 to cost of nutrients in the form of fertilizer is less 1846 was caused by a fungus that produced po- expensive than processing waste to recover and tato blight. Certain species of actinomycetes reuse the nutrients. cause scab in potatoes and sugar beets. The soil used for bedding and greenhouse plants is routinely treated with heat or chemicals to kill plant Mycorrhiza pathogens. Some fungi infect the roots of most plants. Fortu- The organisms that produce disease in humans nately, most of the fungi form a symbiotic rela- must be able to tolerate the soil environment, at tionship-one that benefits both fungi and higher least for a period long enough to cause infection. plants. After the appropriate fungal spores germi- Many disease organisms occur in fecal matter and nate, hyphae, or small segments of the mycelium, human sewage, such as the virus that produces invade young roots and grow a mycelium both on infectious hepatitis and the protozoan (amoeba) the exterior and interior of roots. Fungal hyphae that produces amebic dysentery. When materials on the exterior of roots serve as an extension of containing animal disease organisms are added roots for water and nutrient absorption. These to soil, the organisms encounter a very different external hyphae function as root hairs and are environment than the one they occupied in an called mycorrhiza. Mycorrhiza literally means animal. Consequently, many animal disease or- "fungus roots." SOIL MICROBE AND ORGANISM INTERACTIONS 127

There are two types of hypha or mycorrhiza: tive surface area of roots for the absorption ectotrophic and endotrophic. The ectotrophic hy- of water and nutrients, particularly phosphorus; phae exist between the epidermal cells, using (2) increased drought and heat resistance; and pectin and other carbohydrates for food. Contin- (3) reduced infection by disease organisms. ued growth of the hyphae outside the root results in the formation of a sheath (mantle) that completely surrounds the root, as shown in Figure Nitrogen Fixation 8.10. Literally thousands of species belong in the We live in a "sea" of nitrogen, because the atmo- group of fungi that produce ectotrophic mycor- sphere is 79 percent nitrogen. In spite of this, rhiza and produce mushroom or puffball fruiting nitrogen is generally considered the most limiting bodies. They are associated typically with trees nutrient for plant growth. Nitrogen exists in the air and, thus are beneficial to trees, as illustrated in as N 2 and, as such, is unavailable to higher plants Figure 8.11. and most soil microbes. There are some species Endomycorrhiza are more abundant than ecto- of bacteria (nitrogen fixers) that absorb N 2 gas mycorrhiza. They benefit most of the field and from the air and convert the nitrogen into ammo- vegetable crop plants. Hyphae invade roots and nia that they and the host plant can use. This ramify both between and within cells, usually process of nitrogen fixation is symbiotic. The bac- avoiding the very central core of the roots. The teria obtain food from the host plant and the host host plants provide the fungi with food. The bene- plant benefits from the nitrogen fixed. The bac- fits to the host plant include: (1) increased effec- teria responds to and invades the roots of the host

FIGURE 8.10 Ectomycorrhiza. (a) Uninfected pine root. (b) Infected root with fungal mantle. (c) Diagram of the anatomy. The Hartig net is the hyphae between root cells. (Courtesy D. H. Marx, University of Georgia.) 128 SOIL ECOLOGY

Pesticide Degradation Pesticides include those substances used to control or eradicate insects, disease organisms, and weeds. One of the first successful synthetic pesticides, DDT, was used to kill mosquitoes for ma- laria control. After decades of use it was found in the cells of many animals throughout the world. This dramatized the resistance of DDT to biode- gradation and its persistence in the environment. The structure of DDT appears to be different from naturally occurring compounds and, therefore, few if any organisms have enzymes that can degrade DDT. The difference in structure and de- FIGURE 8.11 Effect of mycorrhizal fungi on the gradability of 2,4-D and 2,4,5-T is illustrated in growth of 6-month-old Monterey pine seedlings. Figure 8.13. (a) Fertile Prairie soil. (b) Fertile Prairie soil plus 0.2 Generally, the pesticides used have almost no percent by weight of Plainfield sand from a forest. significant effect on soil microbes. The soil mi- (c) Plainfield sand alone. Only plants of b and c were infected with mycorrhiza. (Photograph courtesy of the crobes attack the organic pesticides similar to late Dr. S. A. Wilde, University of Wisconsin.) their attack of other organic substrates. In some instances, the microbes detoxify the pesticide, for example, by the shifting of chlorine atoms. In plant, and the host responds by forming a nodule other cases, the pesticide is metabolized or de- that surrounds the bacteria, and in which nitrogen graded. In general, those pesticides that resist is fixed, as shown in Figure 8.12. detoxification and/or degradation and persist in In aquatic ecosystems, blue-green algae fix ni- the environment long after they are applied, are trogen. They are the land counterpart by being the called hard pesticides. The others that degrade major means by which nitrogen from the atmo- easily are soft pesticides. The challenge for scien- sphere is added to aquatic ecosystems. A detailed tists is to develop pesticides that can perform their consideration of nitrogen fixation occurs in Chap- useful function and disappear from the ecoystem ter 12, where the nitrogen cycle is discussed. with minimal undesirable side effects.

Oil and Natural Gas Decontamination SOIL ORGANISMS AND Many species of microbes can decompose petro- ENVIRONMENTAL QUALITY leum compounds. Crude oil and natural gas are a During the billions of years of organic evolution, good energy source, or substrate, for many mi- organisms evolved that could decompose the crobes, and their growth is greatly stimulated. compounds naturally synthesized in the environ- The first result of an oil spill or natural gas leak ment. Today, humans have become important is the displacement of soil air and creation of an contributors of both natural and synthetic com- anaerobic soil. Any vegetation is likely to be killed pounds to the environment. The problems of pes- by a lack of soil oxygen. The reducing conditions ticide degradation and contamination of soils result in large increases in available iron and from oil spills will be considered as they relate to manganese. It is suspected that toxic levels of environmental quality. The application of sewage manganese for higher plants are produced in sludge to the land is covered in Chapter 15. some cases. In time, the oil or natural gas is SOIL ORGANISMS AND ENVIRONMENTAL QUALITY 129

FIGURE 8.12 Symbiotic nitrogen-fixing nodules on the roots of soybean roots (left), and on sweet clover roots (right). (Photographs courtesy Nitragin Company.)

FIGURE 8.13 Structure representing 2,4-D on the decomposed and the soil returns to near normal left and 2,4,5-T on the right. The structures are very conditions. A study of 12 soils exposed to con- similar except for the additional C1 at the meta tamination near Oklahoma City showed that or- position of 2,4,5-T, which is metabolized with great difficulty (if at all) by soil microorganisms. ganic matter and nitrogen contents had been increased about 2.5 times. The increased nitrogen supply in soils that had been contaminated probably explains the increased crop yields obtained on old oil spills. Natural gas leaks are common in cities. The destruction of grass and trees is due to a lack of normal soil air, which is displaced by the natural gas (methane). Sometimes the problem can be diagnosed by detecting the odor of a pungent compound in the natural gas that escapes from a crack in a nearby sidewalk or driveway. Because 130 SOIL ECOLOGY

methane (CH4) is odorless, an odor has been within the soil, depositing it on the surface. Some added to public gas supplies. of the largest ant mounds are about 1 meter tall and more than 1 meter in diameter. The effect of this transport is comparable to that of earthworms in creating thick A horizons and in burying objects EARTH MOVING BY SOIL ANIMALS lying on the soil surface. All soil animals participate as consumers and A study of ant activity on a prairie in southwest- play a minor role in the cycling of nutrients and ern Wisconsin showed that ants brought material energy. Many of the larger animals move soil to to the surface from depths of about 2 meters and such an extent that they affect soil formation. built mounds about 15 centimeters high and more than 30 centimeters in diameter (see Fig. 8.14). Furthermore, it was estimated that 1.7 percent of Earthworm Activity the land was covered with ant mounds. Assuming Earthworms are probably the best known earth that the average life of a mound is 12 years, the movers. Darwin made extensive studies and entire land surface would be reoccupied every found that earthworms may deposit 4 to 6 metric 600 years. The researchers believe that the earth tons of castings per hectare (10 to 15 tons per movement activity of ants resulted in a greater acre) annually on the soil surface. This could than normal content of clay in the A horizon, result in the buildup of a 2.5-centimeter-thick because of the movement to the surface of mate- layer of soil every 12 years. This activity produces rial from a Bt horizon, a clay-enriched B horizon. thicker than normal, dark-colored surface layers Ants also harvest large amounts of plant mate- in soils. Stones and artifacts are buried over time, rial and in some ecosystems are important con- which is a subject of great interest to archae- sumers. Their gathering of seeds retards natural ologists. reseeding on range lands. Leaf harvester ants As a result of their earth-moving activities, march long distances to cut fragments of leaves earthworms leave channels. Where these chan- and stems and bring them to their nests to feed nels are open at the soil surface, they can tranport fungi. The fungi are used as food. Organic matter water very rapidly into and through the soil. Ear- is incorporated into the soil depths and nutrients lier it was reported that infiltration of water from concentrate in the nest sites. irrigation furrows increased after earthworms had Termites inhabit tropical and subtropical soils. burrowed to and entered the irrigation furrows. They exhibit great diversity in food and nesting Renewed interest in the role of earthworms has habits. Some feed on wood, some feed on organic been created in regard to no-till systems, which refuse, and others actively cultivate fungi. Pro- leave plant residues on the soil surface. The resi- tozoa (single-celled microscopic animals) in the dues provide a source of food for the earthworms, digestive tract of many termites aid in the diges- which forage on the soil surface, and the residues tion of woody materials. Some species build huge and growing plants protect the soil surface from nests up to 3 meters high and several meters in raindrop impact so that the channels remain open diameter. Most mounds are on a smaller scale. and functional for water infiltration. Some termites have nests in the soil and tunnnels on the surface to permit foraging for food. Soil material is brought to the surface from depths as Ants and Termites great as 3 meters. Termites have had an enormous The activities of ants and termites are, perhaps, effect on many tropical soils, where their earth- more important than the activities of earthworms. moving activity has been occurring for hundreds Ants transport large quantities of material from of thousands of years. SUMMARY 131

ents occurs at the nest sites, which is why some farmers in Southeast Asia make use of the higher soil fertility in areas occupied by mounds.

Rodents Many rodents, including mice, ground squirrels, marmots, gophers, and prairie dogs inhabit the soil. A characteristic microrelief called mima mounds, which consists of small mounds of earth, is the work of gophers in Washington and California. Mima mounds occur on shallow soils. They appear to be the result of gophers building nests in the dry soil near the tops of the mounds. Successive generations of gophers at the same site create mounds that range from 0.5 to 1 meter high and more than 5 to 30 meters in diameter. Extensive earth moving by paririe dogs has been documented. An average of 42 mounds per hectare (17 per acre) were observed near Akron, Colorado. Each mound consisted of an average of 39 tons of soil material. The upper 2 to 3 meters of the soil formed from loess, a silty wind-deposited material, and was underlain by sand and gravel. All of the mounds observed contained sand and gravel that had been brought up from depths of more than 2 meters. Prairie-dog activity had changed the surface soil texture from silt loam to loam on one third of the area. Abandoned burrows filled with dark-colored surface soil, called crotovinas, are common in grasslands. FIGURE 8.14 Ant (Formica cinera) in a Prairie soil in southwestern Wisconsin. Upper photo shows ant mounds more than 15 centimeters high and more than 30 centimeters in diameter. The lower sketch SUMMARY shows soil horizons and location of ant channels; numbers refer to the number of channels observed at Higher plants are the major producers contribut- the depth indicated. (Photographs courtesy Dr. F. D. ing to the supply of soil organic matter. The mi- Hole, Wisconsin Geological and Natural History croorganisms (bacteria and fungi) are the major Survey, University of Wisconsin.) decomposers and are mainly responsible for the cycling of nutrients and energy in soil eco- In summary, ants and termites create channels systems. in soils and transport soil materials to such an Soil animals play a minor role in the cycling of extent that soil horizons are greatly affected and nutrients and energy, but play an important role in sometimes obliterated. A concentration of nutri- earth-moving activities. 132 SOIL ECOLOGY

Nutrient cyling results in reuse of the nutrients Agriculture." Adv. in Agronomy. 13:249-268. Aca- in an ecosystem. Nutrients are efficiently recycled demic Press, New York. in natural ecosystems. Interference of the cycle, Baxter, F. P. and F. D. Hole. 1967. "Ant Pedoturbation in such as cropping and removal of nutrients in a Prairie Soil." Soil Sci. Soc. Am. Proc. 31:425-428. Bormann, F. F. and G. E. Likens. 1970. "The Nutrient food, results in reduced soil fertility. Manures and Cycles of an Ecosystem." Sci. Am. 223:92-101. fertilizers are used to maintain soil fertility in agri- Bowen, E. 1972. "The High Sierra." Time, Inc., New culture. York Soil organisms and higher plants engage in Buol, S. W., F. D. Hole, and R. J. McCracken. 1973. Soil many interactions related to disease, mycorrhiza, Genesis and Morphology. I owa State University and nitrogen fixation, and soil organisms and Press, Ames. higher plants compete for the same growth Ellis, R., Jr. and R. S. Adams, Jr. 1961. "Contamination factors. of Soils by Petroleum Hydrocarbons." Adv. in A zone adjacent to plant roots with a high popu- Agronomy. 13:197-216. Academic Press, New York. lation of microorganisms is the rhizosphere. Flaig, W., B. Nagar, H. Sochtig, and C. Tietjen. 1977. Microorganisms play important environmental Organic Materials and Soil Productivity. FAO Soils Bull. 35. United Nations, Rome. quality roles, such as detoxification of chemicals Richards, B. N. 1974. Introduction to the Soil Ecosys- and decomposition of oil from spills. tem. Longmans, New York. Earthworms, ants, termites, and rodents move Thorp, J. 1949. "Effect of Certain Animals that Live in large quantities of soil and may greatly alter the Soils." Sci. Monthly. 42:180-191. nature of soil horizons. Schaller, F. 1968. Soil Animals. University of Michigan Press, Ann Arbor. Smucker, A. J. M. 1984. "Carbon Utilization and Losses by Plant Root Systems." Roots, Nutrient and Water REFERENCES Flux and Plant Root Systems. ASA Spec. Pub. 49, Soil Sci. Soc. Am., Madison. Alexander, M. 1977. Soil Microbiology, 2nd ed. John Stoeckeler, J. H. and H. F. Arneman. 1960. "Fertilizers in Wiley, New York. Forestry." Adv. in Agronomy. 12:127-195. Arkley, R. J. and H. C. Brown. 1954. "The Origin of Mima Temple. K. L. A. K. Camper, and R. C. Lucas. 1982. Mound Microrelief in the Far Western States." Soil "Potential Health Hazard from Human Wastes in Wil- Sci. Soc. Am. Proc. 18:195-199. derness." J. Soil and Water Con. 37:357-359. Barley, K. P. 1961. "The Abundance of Earthworms in Walters, E. M. P. 1960. Animal Life in the Tropics. Agricultural Land and Their Possible Significance in George Allen and Unwin, London. CHAPTER 9

SOIL ORGANIC MATTER

Almost all life in the soil is dependent on organic there is a continual growth of plants and additions matter for nutrients and energy. People have long to these three pools of organic matter: standing recognized the importance of organic matter for crop, forest floor, and soil. These three pools of plant growth. Although organic matter in soils is organic matter represent a supply of nutrients for very beneficial, Justus von Liebig, a famous Ger- future growth. man chemist, pointed out about 150 years ago In grassland ecosystems, much more of the that soils composed entirely of organic matter are organic matter is in the soil and much less occurs naturally infertile. This chapter considers the ori- in the standing plants and grassland floor. Al- gin, nature, and importance of the organic matter though approximately 50 percent of the total or- in soils. ganic matter in the forest ecosystem may be in the soil, over 95 percent may be in the soil, where grasses are the dominant vegetation.

THE ORGANIC MATTER IN ECOSYSTEMS The organic matter in an ecosystem consists of DECOMPOSITION AND ACCUMULATION the organic matter above and below the soil surface. The distribution of organic matter in a pon- The primary or original source of soil organic derosa pine forest ecosystem is shown in Table matter (SOM) is the production of the primary 9.1. The distribution of organic matter, expressed producers-the higher plants. This organic mate- as organic carbon, was 38 percent in the trees and rial is subsequently consumed and decomposed ground cover and 9 percent in the forest floor. The by soil organisms. The result is the decom- remaining 53 percent of the organic matter was in position and the accumulation of organic matter the soil and included the roots plus the organic in soils that has great diversity and a highly vari- matter associated with soil particles. In the forest able composition.

133 134 SOIL ORGANIC MATTER

TABLE 9.1 Quantity and Distribution of Organic Carbon (organic matter) in a Ponderosa Ecosystem in Arizona Organic Carbon

Adapted from Klemmedson, 1975.

Decomposition of Plant Residues ble fraction, cellulose and hemicellulose, and The plant residues, including tops and roots, con- other readily degradable compounds, there was a tain a wide variety of compounds: sugar, cellu- corresponding and rapid increase in microbial lose, hemicellulose, protein, lignin, fat, wax, and products (see Figure 9.1b). others. Observe from Table 9.2 that the composi- Microbial products include living and dead mi- tion of SOM is distinctly different than that of the crobial cells and their waste or excretion prod- original plant material added to the soil. ucts. Some of the organic compounds that are In an experiment, wheat straw was added to synthesized in the soil during decomposition re- soil and the changes in the major components in act with each other and with mineral soil com- the straw were followed over time. It was found ponents. Consequently, the decomposition of that proteins, the soluble fraction, and the cellu- plant residues results in: (1) the production of lose and hemicellulose disappeared or decom- a considerable mass of microbial products, and posed very rapidly, whereas lignin decomposed (2) the production of a wide variety of materials of very slowly, as shown in Figure 9.1a. As a result of varying resistance to decomposition. As a result the rapid decomposition of proteins and the solu- of the addition and decomposition of plant and animal residues in soils, labile and stable fractions of organic matter are produced. Labile or- TABLE 9.2 Partial Composition of Mature Plant ganic matter is organic matter that is liable to Tissue and Soil Organic Matter change or is unstable. The liable and stable organic matter fractions correspond, in general, to the organic residues and humus fractions, respectively.

Labile Soil Organic Matter The labile fraction of SOM consists of any readily degradable materials from the plant and animal residues, and readily degradable microbial prod- DECOMPOSITION AND ACCUMULATION 135

FIGURE 9.1 Decomposition of wheat straw in soil in the laboratory. (a) Decreases over time: proteins and solubles (C1), cellulose and hemicellulose (C2), and lignin (C3). (b) Changes over time in amount of carbon in original plant material and in microbial products. (Data from Van Veen and Paul, 1981.) ucts. The labile organic matter composes about studies have shown that certain stable SOM frac- 10 to 20 percent of the total SOM. Labile organic tions are commonly more than a few hundred or matter is an important reservoir of nutrients be- thousands of years old. The stable organic matter cause the nutrients, especially nitrogen, are rap- has a long residence time in the soil and plays idly recycled in the soil ecosystem. The addition important roles in structure formation and sta- to the soil of a relatively small amount of easily bility, water adsorption, and adsorption of nutri- degraded organic matter, such as manure, or- ent cations. ganic waste materials, and plant residues, may Stable soil organic matter, or humus, originates contribute more to the available nutrient supply from several processes. Lignin, a 6-carbon ring than the slow release of nutrients from the much structure, is a plant compound and is very resis- larger amount of stable organic matter. The labile tant per se. Thus, lignin accumulates in soils be- organic matter is rapidly degraded when condi- cause of its resistant or stable nature. Organic tions are favorable. As a result, any labile organic compounds of varying resistance interact with lig- matter incorporated within soils tends to disap- nin and form new compounds that are resistant. pear quickly if conditions are favorable for de- This accounts for the significant quantity of composition. Therefore, the decomposers are fre- protein or proteinlike material in humus (see Ta- quently quite inactive, owing to the rapid ble 9.2). disappearance of readily decomposable sub- Much of the stable organic matter is intimately strates or labile organic matter. associated with the mineral particles. As such, stable organic matter plays an important role in the formation of soil structure and the ability of Stable Soil Organic Matter soil peds to resist crushing and breakdown from The stable SOM fraction consists of resistant com- tillage and raindrop impact. Studies have shown pounds: (1) in the decomposing residues, (2) in that some readily decomposable materials microbial products, and (3) that formed as a re- (sugars and proteins) are decomposed by mi- sult of interaction of organic compounds with crobes in less than a day under laboratory condi- each other and with the mineral components of tions. When the same materials are added to the soil, especially the clay. The stable organic soils, the decomposition time is greatly in- matter is equivalent to humus. The nutrients creased. The adsorption of organic compounds within it are recycled very slowly. Radioactive onto soil, particles makes these compounds less 136 SOIL ORGANIC MATTER decomposable. Scientists believe that the organic association of mineral and organic matter that matter is protected from decomposition; this protects the organic matter from decomposition protection helps to increase the stability of both and forms stable soil peds. One half or more of readily decomposable and resistant organic mat- the SOM is likely to be protected. ter. Much of the humus is associated intimately with clay and silt particles, and the humus plays Decomposition Rates an important role in formation of soil structure, as Although SOM has been characterized as having shown in Figure 9.2. two forms-labile and stable-there is great vari- Polysaccharides include microbial gum and ety in the nature and decomposition rates of vari- two other abundant compounds found in micro- ous organic fractions of the SOM. In a native bial cell walls: hemicellulose and cellulose. grassland soil, about 9 percent of the SOM.were These polysaccharides are organic polymers (gi- shown to consist of roots and plant residues and 4 ant organic moleclules formed by the combina- percent consists of unprotected and decomposa- tion of smaller molecules) having high molecular ble liable organic matter with a decomposition weight and are present as ropes and nets. These rate of 8 percent per day. This rate, which as- polymers are widely and intimately associated sumes optimal conditions, is shown in Table 9.3. with the soil fabric. On drying, soil particle sur- Soil organisms were 1.3 percent of the total or- faces are brought into very close contact with the ganic matter with a decomposition rate of 3 per- netlike arrangement of the organic compounds. cent per day. The hydroxyls of the polymers and the exposed The other fractions of the SOM, 84 percent, had oxygen atoms of the mineral soil particles bind much slower decomposition rates. The bulk of strongly to each other. The result is an intimate the SOM (54 percent) consisted of decomposable organic matter that was protected by attachment to clays and other mineral components, and had a FIGURE 9.2 Some arrangements of organic matter decomposition rate of 0.08 percent per day. Two and mineral soil particles. Organic matter intimately fractions, composed mainly of ligneous material, associated with mineral particles is protected from decomposed at 0.0008 and 0.000,008 percent per decomposition and contributes to the formation and stability of soil structure. (a) Quartz-organic day, for unprotected and protected ligneous mate- matter-quartz. (b) Quartz-organic matter-clay. rials, respectively. Thus, the decomposition rate (c) Organic matter-clay-organic matter. of the most readily decomposable material is about 1 million times that of the most stable (see Table 9.3). The most resistant organic matter fractions have been called recalcitrant, meaning stubborn. Assuming that plant residues, roots, decomposable unprotected organic matter, and soil organisms are liable organic matter, the data in Table 9.3 show that the soil contained about 15 percent liable organic matter and 85 percent stable organic matter.

Properties of Stable Soil Organic Matter Stable soil organic matter (humus) is a heterogeneous mixture of amorphous compounds that are resistant to microbial decomposition and possess DECOMPOSITION AND ACCUMULATION 137

TABLE 9.3 Soil Organic Matter and Decomposition Rates under Optimum Conditions for a Native Grassland Soil

Organic carbon, upper 16 inches

Adapted from Van Veen and Paul, 1981.

a large surface area per gram. The large surface negative charge. Cations in the soil solution tend area enables humus to absorb water equal to to be weakly adsorbed onto the negatively many times its weight. charged phenolic and carboxyl groups. However, During decomposition of plant and animals adsorbed cations can be readily exchanged by residues, there is a loss of carbon as carbon diox- other cations remaining in solution. The total neg- ide and the conservation and reincorporation of ative charge of the SOM, or ability to adsorb ex- nitrogen into microbial products, which eventu- changeable cations, is called the cation exchange ally are incorporated into humus. As a conse- capacity (CEC). The CEC of organic matter acts quence, humus contains about 50 to 60 percent similarly to that of clay particles, in that the charge carbon and about 5 percent nitrogen, producing a adsorbs and holds nutrient cations in position to C : N ratio of 10 or 12 to 1. The more decomposed be available to plants, while effectively holding the humus is the lower or narrower will be the these cations against loss by leaching. The most C : N ratio. Compared with plant materials, hu- abundant nutrient ions adsorbed are Ca.²1 , M g ²+ , + mus is relatively rich in nitrogen and, when miner- and K . Adsorbed H + affects the pH of the soil alized, humus is a good source of biologically solution, depending on the amount adsorbed and available nitrogen. Humus is also a significant the degree of dissociation into the soil solution. source of sulfur and phosphorus. The ratio of Clay and SOM are the source of most of the C:N:P:S is about 100:10:1:1. negative charge in soils. The sum of the CEC of the Humus contains phenolic groups, 6-C rings SOM plus the CEC of the clay is generally con- with OH, that dissociate H + at pH above 7.0. sidered equivalent to the CEC of the total soil. The During humus formation there is an increase in source and amount of CEC of soil clays are dis- carboxyl groups, R-COOH, and these groups tend cussed in Chapters 10 and 11. to dissociate H+ at pH less than 7:

- R-COOH = R-C00 + H+ Protection of Organic Matter b Cla The amount of organic matter present in a soil at As a result, humus has a considerable amount of any given time is the net effect of the amount that 138 SOIL ORGANIC MATTER has been added minus the amount that has been decomposed. The major factors affecting the SOM content are soil texture, vegetation type, water content or aeration of the soil, and soil temperature. Locally, within a given landscape, a correlation has been found between soil texture and the organic matter content of soils as a result of the role of the soil matrix, especially clay, in protecting organic matter from decomposition. Organic matter is positively correlated with the clay content and negatively correlated with the sand content in some grassland soils in Wyoming (see Figure 9.3). These soils had a mean average soil temperature of 8 to 15° C (47 to 59° F). In the same study, soils with lower average FIGURE 9.4 Model of soil nitrogen variation of the temperature showed a weaker relationship be- Great Plains from Canada to Mexico (back to front tween organic matter content and texture. This curves) and from the desert to the humid regions suggests that low soil temperature, by decreasing (left to right curves). The NSQ is the precipitation the rate of decomposition, appeared to have had divided by the absolute saturation deficit of air; it is a an important effect on the organic matter content measure of the effectiveness of the precipitation. Decreases in temperature and increases in humidity of the soils. The data in Figure 9.4 are generally are associated with increases in total soil-nitrogen representative of the plains of the United States content. (From Jenny and used by permission of between Canada and Mexico, and show that high Springer-Verlag, New York, copyright (D 1980.) precipitation at low temperature results in much more organic matter in soils as compared to high precipitation and high temperature. In most local ble clay content tend to have a higher content of (farm) situations, where temperature is a con- organic matter and, therefore tend to have a good stant, clay content is positively associated with combination of soil fertility and physical organic matter content. Thus, soils with apprecia- properties.

FIGURE 9.3 Relationships between the content of organic matter to a depth of 40 centimeters and the clay and sand content of some Wyoming grassland soils. (Adapted from McDaniel and Munn, 1985.) ORGANIC SOILS 139

ORGANIC SOILS ganic soil materials that are of a significant thickness, depending on the depth to rock and other The emphasis thus far has been on mineral soils, lithic materials. soils with a volume composition consisting of about 50 percent solids, and these solids composed of 45 percent mineral matter and 5 percent Formation of Organic Soils organic matter. In such soils the mineral fraction High organic matter contents in soils are the re- has a dominating influence on soil properties. sult of slow decomposition rates rather than high Organic soils, by contrast, have properties that are rates of organic matter addition. Organic soils are controlled mainly by the organic matter. very common in tundra regions where the rate of plant growth is very slow. In the shallow water of lakes and ponds, plant residues accumulate in- Organic Soil Materials Defined stead of decomposing because of the anaerobic Organic soil materials are saturated with water for conditions. Consequently, organic soils may con- long periods or are artificially drained and, ex- sist almost entirely of organic matter. In some cluding roots, (1) contain 18 percent (dry-weight cases, an infusion of mineral sediment from soil basis) organic carbon if the mineral fraction is 60 erosion may result in the accumulation of some percent or more clay; (2) contain 12 percent or mineral matter, together with the organic matter. more organic carbon if the mineral fraction has no Some organic soils, peat, for example, are mined clay; and (3) contain a proportional content of and used for fuel. organic carbon between 12 and 18 percent, de- Formation of many organic soils consists of the pending on clay content, as shown in Figure 9.5. If accumulation of organic matter in a body of wa- the soils are never water saturated for more than a ter. After glaciation, many depressions received few days each year, the organic carbon content clay and other materials that sealed the bottoms must be 20 percent or more. and created ponds and lakes. As the plant mate- Organic soils are those soils composed of or- rial accumulated on the bottom of the bodies of

FIGURE 9.5 Organic carbon content (dry weight basis) of organic and mineral soil materials. Mineral soil materials below the heavy solid line and organic soil materials above the heavy solid line. Organic soil materials above the dashed line are water saturated for no more than a few days per year. 140 SOIL ORGANIC MATTER water, the water depth decreased and the kinds of plants contributing organic matter changed. An- other important organic component is the remains and fecal products of small aquatic animals. Minor climatic changes have occurred since the last glaciation, and this has had an effect on the water levels and types of vegetation and the animal populations. Thus, a series of layers or soil horizons develop from the bottom upward that reflect the conditions at the time each layer was formed. Pollen grains are preserved in the peat, which can be readily identified. The vegetation type can be verified by pollen analysis. A total of 5 meters of peat soil accumulated in Sweden in 9,000 years or at a rate equal to 1 centimeter every 18 years. This is illustrated in Figure 9.6.

Properties and Use Organic soils are composed mainly of plant and animal residues and some microbial decomposition products. The soils are very light and have low bulk density, high CEC, and a high nitrogen, phosphorus, and sulfur content.The absence or low content of sand and silt particles, composed of minerals containing essential nutrients, FIGURE 9.6 Stratification found in a peat soil in may result in low amounts of available potassium. Sweden, where changes in climate produced changes in the kind of vegetation that formed the peat. (Data Organic soils must be drained before they can from Lucas, 1982.) be used for crop production. The upper soil layers then become aerobic and the organic matter begins to decompose. This converts undecomposed peat into well-decomposed organic soil called Archaeological Interest muck. In time, the entire soil may disappear. This The excellent preservation ability in a peat bog is is a serious problem in a warm climate with a illustrated by examination of the famous Tollund rapid decomposition rate, as in the Florida Ever- man, who was found in 1950 in a peat bog in glades, where sugarcane is a major crop, together Denmark. The facial expression at death and with a wide variety of vegetable crops (see Figure bristles of the beard were well perserved. Excel- 9.7). Organic soils typically occupy the lowest lent fingerprints were made, and an autopsy elevation in the landscape, and crops grown in revealed the contents of the last meal, which con- such soils are the most likely to be killed by frost. sisted mainly of seeds, many of them weed seeds. Their low bulk density makes them susceptible to When found, the Tollund man was buried under 2 wind erosion and desirable for sod production meters of peat that was estimated to have formed (see Figure 9.8). in the 2,000 years since his burial. THE EQUILIBRIUUM CONCEPT 141

FIGURE 9.7 The subsidence rates for organic soils depends on the depth to the water table; the lower the water table the greater is the soil aeration and decomposition of organic matter. Subsidence is greater in Florida than in Indiana because of higher temperatures in Florida.

THE EQUILIBRIUUM CONCEPT A Case Example During the early phase of soil formation, soil or- The pool, or amount of organic matter in a soil, ganic matter content increases. After a few centu- can be compared to a lake. Changes in the water ries, the soil attains a dynamic equilibrium con- level in a lake depend on the difference between tent of organic matter; additions balance losses. the amount of water entering and leaving it. This Even though the organic matter content remains idea, applied to soil organic matter, is illustrated constant from year to year, there is a significant at the top of Figure 9.9, showing that the change in turnover of organic matter and recycling of nu- soil organic matter is the difference between the trients. amount of organic matter added and the amount that is decomposed. FIGURE 9.8 Organic soil landscape. Crop on left is Soil organic matter, labile and stable con- grass sod used for landscaping and in the rear is a sidered together, decomposes in mineral soils at windbreak of willow trees. The lightness of the soil a rate equal to about I to 3 percent per year. makes the soil desirable for sod production; however, it also contributes to susceptibilty to wind Assuming a 2 percent decomposition rate, and erosion. 40,000 kilograms in the plow layer of a hectare, 800 kilograms of soil organic matter would be decomposed or lost each year. Conversely, if 800 kilograms of humus or stable organic matter were formed from decomposition of residues added to the soil, the organic matter content of the soil would remain the same from one year to the next. The soil would be at the equilibrium level illustrated in Figure 9.9. Decomposition of 800 kilograms of organic matter containing 5 percent nitrogen would result in mineralization of 40 kilograms per hectare of nitrogen. On an acre basis, this would amount to about 40 pounds of available nitrogen per acre. 142 SOIL ORGANIC MATTER

FIGURE 9.9 Schematic illustration of the equilibrium concept of soil organic matter as applied to a hectare plow layer weighing 2 million kilograms and containing 2 percent organic matter. (Values are similar on a pounds per acre basis for a 2-million pound acre-furrow-slice.)

Assuming that the ratio of N:P:S in the soil organic At the Missouri Agricultural Experiment Station, matter is 10:1:1, there would be 4 kilograms per as a result of 60 years of cultivation, the soil lost hectare (4 lb/acre) of phosphorus and sulfur min- one third of its organic matter (see Figure 9.10). eralized. Other nutrients are also mineralized, but The organic matter content decreased 25 percent their availability is more closely related to the soil during the first 20 years, 7 percent during the next mineral fraction and mineral weathering. 20 years, and only 3 percent during the third 20- year period. The organic matter content of the soil was moving to a new equilibrium level. In general, Effects of Cultivation cultivated soils today contain about one half as Even on nonerosive land that is converted from much organic matter as they did before culti- grassland or forest to cultivated land, a rapid loss vation. Theoretically, if a soil is fallowed, so that of organic matter resulting from cropping occurs. no plants grow and decomposition continues, the THE EQUILIBRIUUM CONCEPT 143

the SOM content. In this case, converting land to agricultural use results in an equilibrium level that is higher than that of the virgin land.

Maintenance of Organic Matter in Cultivated Fields The organic matter content of a soil is more difficult to determine than the carbon content. As a result, changes in the soil carbon content are used as a measure of changes in organic matter FIGURE 9.10 Decline in soil nitrogen (or organic content. Multiplying the percent of soil carbon by matter) with length of time of cultivation under average farming practices in Missouri. (Data from 1.72 is generally considered to equal the percent Jenny, Missouri Agr. Exp. Sta. Bull. 324, 1933.) of organic matter. Soil organic matter is constantly subject to decomposition and loss. Therefore, each soil and organic matter content would eventually ap- cropping situation, requires the addition of a cer- proach zero. On the other hand, if cultivated fields tain amount of organic matter each year to main- are allowed to revert to their original native vege- tain the status quo. In a 16-year experiment in tation, the organic matter content will gradually Minnesota, 5 tons of crop residues per acre annu- increase toward the equilibrium level that existed ally were required to maintain the original carbon prior to cultivation. content of the soil at 1.8 percent (see Figure 9.11). Soils in arid regions naturally have a low or- Addition of more than 5 tons per acre increased ganic matter content. Irrigating such soils and SOM content and lesser amounts resulted in a producing crops that return a large amount of decline in the organic matter content. While land plant residues to the soil results in an increase in is being farmed (excluding the desert land), it is

FIGURE 9.11 Relationship between annual application rates of crop residues and content of organic carbon in soils after 11 years. (Data of Frye, Bennett, and Buntley, 1985). 144 SOIL ORGANIC MATTER virtually impossible, to maintain the organic mat- kilograms per hectare would increase the soil carter content of the virgin soil. It is, therefore, bon content by about 800 kilograms per hectare. prudent to accept the organic matter content that Since World War II there have been large in- results from economical or profitable farming. creases in crop yields in many countries in spite In Figure 9.12, the effect of different yields of of claims of declining soil organic matter and soil corn grain on soil carbon is given for a typical depletion. It appears that soil management sys- well-drained agricultural soil in southern Michi- tems that result in high crop yields maintain SOM gan. The grain was harvested and all of the re- at acceptable contents. Many soil management maining crop residues were returned to the soil, programs focus on the production of high crop including the roots that equal about 15 to 20 per- yields and proper crop residue management cent of the total crop residues. Note that a corn rather than the organic matter content of soils, per grain yield of 3,150 kilogram per hectare (kg/ha) se. One of the most serious causes of organic (50 bushels per acre) is predicted to maintain an matter loss is soil erosion that tends to prefer- equilibrium level equal to 1.0 percent organic car- entially remove the organic matter fraction. High bon. By contrast, yields of 6,300 and 9,450 kilo- crop yields also mean more vegetative cover and grams per hectare (100 and 150 bushels per acre) reduced soil erosion. are predicted to maintain equilibrium carbon levels of 1.7 and 2.3 percent, respectively. Higher yields return more residues and maintain a higher Effects of Green Manure organic matter content, which in turn makes still Green manuring is the production of a crop for the higher yields more likely. purpose of soil improvement. These crops are Another observation can be made from Figure easily and inexpensively established and, fre- 9.12. A yield of 6,300 kilograms per hectare would quently, are grown during the fall, winter, and/or spring result in about a 400 kilograms per hectare loss of when the land would normally be un- carbon if the soil contained 2.5 percent organic protected by vegetation. Benefits include erosion carbon. Conversely, if the soil contained only 0.5 control and the accumulation of nutrients (into percent organic carbon, the same yield of 6,300 organic matter) that might otherwise be lost by leaching. There is also less likelihood of pollution of water tables with nitrate nitrogen. In addition, FIGURE 9.12 Estimated annual soil-humus carbon organic matter from the green manure crop is changes as related to corn (maize) yields and eventually incorporated into the soil, and the fol- organic carbon content of loamy soils in southern l Michigan. (Adapted from Lucas and Vitosh, 1978.) owing crop benefits from the rapid release of nutrients from a large amount of readily decomposable (labile) material. The humus that is formed contributes to the maintenance of the content of soil organic matter. However, this is usually a secondary reason why green manure crops are grown.

HORTICULTURAL USES OF ORGANIC MATTER Organic materials used for horticultural purposes consist mainly of naturally occurring peat and moss and the products of composting. High- HORTICULTURAL USES OF ORGANIC MATTER 145 content organic matter materials are used as a component of soil mixes and as a mulch.

Horticultural Peats Peats that are used for soil amendments are generally classified as moss peat, reed-sedge peat, and peat humus. Moss peat forms from moss vegetation and reed-sedge peat from reeds, sedges, cattails, and other associated plants. Some properties of common horticultural peats are given in Table 9.4. The low nitrogen content of sphagnum peat means that it may not have FIGURE 9.13 Mulching a rose garden with peat enough nitrogen to satisfy the needs of decom- moss. The mulch protects the soil surface from raindrop impact and maintains a high posers and, thus, its use creates competition be- water-infiltration rate. tween decomposers and plants for any available nitrogen that might be present in the soil mix. The low pH of sphagnum peat, however, makes it pings, or other plant and garbage wastes. These desirable as a mulch for plants, such as azaleas materials can be composted with a resulting de- and rhododendrons, that grow well in acid soil. crease in the C:N ratio and production of an or- Peat moss makes a neat-looking surface that sets ganic material used for mulching and as a com- off plants and protects the soil from the disruptive ponent of soil mixes. Composting consists of impact of raindrops. The soil remains more po- storing or maintaining the organic materials in a rous and receptive to rain or irrigation water (see pile while providing favorable water content, aer- Figure 9.13). ation, and temperature. As the organic matter decomposes, much of the carbon, hydrogen, and Composts oxygen is released as carbon dioxide and water. Many gardners have organic residues with wide Nutrients, like nitrogen, are continuously reused C : N ratios such as tree leaves, weeds, grass clip- and recycled by the microbes and conserved

TABLE 9.4 Characteristics of Common Horticultural Peats

From Lucus, et al., Ext. Bull., 516, Micoigan State University, East Lansing, Micoigan. a Oven dry basis. 146 SOIL ORGANIC MATTER

TABLE 9.5 Materials Recommended for microbial, and animal products. Even though it is Making Compost a minor amount of the total soil organic matter, labile organic matter is very important in nutrient cycling. The stable organic matter is humus, which contributes to cation exchange capacity, water adsorption, and soil structure stability. Sta- ble organic matter decomposes very slowly, and as a result, releases (mineralizes) nutrients very slowly. Organic soils consist mainly of organic matter. They have light weight and are susceptible to wind erosion. Most organic soils require drainage for cropping, which aerates the soil and greatly increases the rate of decomposition. The absence or very low content of minerals is associated with very low potassum supplying power. The organic matter content of a soil tends to approach an equilibrium. When the losses of organic matter exceed additions, the soil organic matter content declines. Cultivated soils contain From Kellogg, 1957. about half as much organic matter as they did before the soils were converted to cropping. The within the composting pile. Thus, while there is a exception is desert soils where cropping may re- loss of carbon, oxygen, and hydrogen, other nutri- sult in an increase in organic matter content. ent elements are being concentrated. The occa- Horticultural peats and composts are organic sional turning of the compost pile increases aer- materials used for mulching and for soil mixes. ation and generally hastens rotting. The rotted material has good physical properties and is a good source of nutrients. Small amounts of mate- REFERENCES rials, from the weeding of a garden, for example, Alexander, M. 1977. Soil Microbiology. 2nd ed. John can be composted in a black plastic bag. Wiley, New York. The low nitrogen content of many composting Allison, F. E. 1973. Soil Organic Matter and Its Role in materials may retard their decomposition. For this Crop Production. Elsevier, New York. reason, most composters add some nitrogen fer- Frye, W. W., 0. L. Bennett, and G. J. Buntley. 1985. tilizer to the compost pile. The nutritive value of "Restoration of Crop Productivity on Eroded or De- the compost can also be increased by the addition graded Soils." Soil Erosion and Crop Productivity. pp. of other materials. Some recommendations of 335-356. Am. Soc. Agronomy. Madison, Wis. the U.S. Department of Agriculture are given in Ta- Glob, P. V. 1954. "Lifelike Man Preserved 2,000 Years in ble 9.5. Peat." Nat. Geog. Mag. 105:419-430. Jenny, Hans. 1933. "Soil Fertility Losses Under Missouri Conditions." Missouri Agr. Exp. Sta. Bull. 324. Colum- bia, Mo. SUMMARY Jenny, Hans. 1980. The Soil Resource. Springer-Verlag, New York. The total soil organic matter is composed of a Kellogg, C. E. 1957. "Home Gardens and Lawns". USDA labile fraction and a stable fraction. The labile Agricultural Yearbook. pp. 665-688. Washington, fraction consists of readily decomposable plant, D.C. REFERENCES 147

Klemmedson, J. O. 1975. "Nitrogen and Carbon McDaniel, P. A. and L. C. Munn. 1985. "Effect of Tem- Regimes in an Ecosystem of Young Dense Pon- perature on Organic Carbon-texture Relationships in derosa Pine in Arizona." Forest Science. 21:163- Mollisols and Aridisols." Soil Sci. Soc. Am. J. 49: 168. 1486-1489. Lucas, R. E. 1982. "Organic Soil (Histosols)." Mi. Agr. Paul, E. A. 1984. "Dynamics of Organic Matter in Soils." Exp. Sta. Res. Report 435. East Lansing, Mi. Plant and Soil. 76:275-285. Lucas, R. E. and P. E. Rieke, and R. S. Farnham. 1971. Schreiner, O. and B. E. Brown. 1938. "Soil Nitrogen." "Peats for Soil Improvement and Soil Mixes." Mi. Agr. Soils and Men. USDA Yearbook of Agriculture, Wash- Ext. Bull. 516. East Lansing, Mi. ington, D.C. Lucas, R. E. and M. L. Vitosh. 1978. "Soil Organic Matter Van Veen, J. A. and E. A. Paul. "1981. Organic Carbon Dynamics." Mi. Agr. Exp. Sta. Res. Report 358. East Dynamics in Grassland Soils." Can. J. Soil Sci. Lansing, Mi. 61:185-210. CHAPTER 10

SOIL MINERALOGY

Minerals are natural inorganic compounds, with form to another, is essential to understanding definite physical and chemical properties, that are both the nature of the soil's chemical properties so conspicuous in many granitic rocks. They are and the origin of its fertility. Since soils develop broadly grouped into primary and secondary min- from parent material composed of rocks and minerals. Primary minerals have not been altered erals from the earth's crust, attention is directed chemically since their crystallization from molten first to the chemical and mineralogical composi- lava. Disintegration of rocks composed of primary tion of the earth's crust. minerals (by physical and chemical weathering) releases the individual mineral particles. Many of these primary mineral particles become sand and CHEMICAL AND MINERALOGICAL silt particles in parent materials and soils. Primary COMPOSITION OF THE EARTHS CRUST minerals weather chemically (decompose) and About 100 chemical elements are known to exist release their elements to the soil solution. Some in the earth's crust. Considering the possible com- of the elements released in weathering react to binations of such a large number of elements, it is form secondary minerals. A secondary mineral not surprising that some 2,000 minerals have results from the decomposition of a primary min- been recognized. Relatively few elements and eral or from the precipitation of the decom- minerals, however, are of great importance in position products of minerals. Secondary miner- soils. als originate when a few atoms react and precipitate from solution to form a very small crystal that increases in size over time. Because of the Chemical Composition of the generally small particle size of secondary miner- Earths Crust als, they dominate the clay fraction of soils. Approximately 98 percent of the mass of the Consideration of the characteristics of minerals earth's crust is composed of eight chemical ele- found in soils, and their transformation from one ments (see Figure 10.1). In fact, two elements,

148 WEATHERING AND SOIL MINERALOGICAL COMPOSITION 149

are mined; in other areas, there is a deficiency of phosphorus for plant growth.

Mineralogical Composition of Rocks Most elements of the earth's crust have combined with one or more other elements to form the minerals. The minerals generally exist in mixtures to form rocks, such as the igneous rocks, granite and basalt. The mineralogical composition of igneous rocks, and the sedimentary rocks, shale and sandstone, are given in Table 10.1. Limestone is also an important sedimentary rock, which is composed largely of calcium and magnesium carbonates, with varying amounts of other minerals as FIGURE 10.1 The eight elements in the earth's crust impurities. The dominant minerals in these rocks comprising over 1 percent by weight. The remainder are feldspars, amphiboles, pyroxenes, quartz, of elements make up 1.5 percent. (Data from Clark, mica, apatite, clay, iron oxides (goethite), and 1924.) carbonate minerals. oxygen and silicon, compose 75 percent of it. Many of the elements important in the growth of WEATHERING AND SOIL plants and animals occur in very small quantities. MINERALOGICAL COMPOSITION Obviously, these elements and their compounds are not evenly distributed throughout the earth's The soil inherits from the parent material a min- surface. In some places, for example, phosphorus eral suite (a set of minerals) that typically con- minerals (apatite) are so concentrated that they tains both primary and secondary minerals.

TABLE 10.1 Average Mineralogical Composition of Igneous and Sedimentary Rocks

° Present in small amounts. 150 SOIL MINERALOGY

Weathering in soils results in the destruction of leached from the soil, particularly in humid re- existing minerals and the synthesis of new miner- gions. als. The minerals with the least resistance to The kaolinite, even though very resistant to fur- weathering disappear first. The minerals most re- ther weathering, may be decomposed and disap- sistant to weathering such as quartz, make up a pear from the soil. The reaction: greater proportion of the remaining soil and appear to accumulate. As a consequence, the mineralogy of a soil changes over time, when viewed on a time scale of hundreds of thousands of years. The decomposition of kaolinite resulted in the formation of gibbsite, a mineral more resistant to weathering than the kaolinite. Both kaolinite and Weathering Processes gibbsite are clay minerals that have distinctly dif- Numerous examples of weathering abound and ferent properties. Thus, as weathering proceeds can be observed every day. These include the and the mineralogical composition of the soil rusting of metal and the wearing down and disin- changes, the chemical and physical properties tegration of brick walls. Mineral weathering is also change. stimulated by acid conditions. Respiration of The loss of silicic acid by leaching results in the roots and microorganisms produces carbon diox- progressive loss of silicon. There is a progressive ide that reacts with water to form carbonic acid increase in the accumulation of aluminum, because the aluminum tends to be incorporated into (H ² C0 3). An acid environment stimulates the reaction of water with minerals and is one of the resistant secondary minerals that accumulate in most important weathering reactions. For exam- the soil. For this reason, an indicator of weather- ple, the reaction of a feldspar (albite) with water ing intensity is the silica:alumina ratio (Si0 ² / (hydrolysis) and H + is as follows: A1 ² 0 3). As silicon is lost from the soil by leaching and aluminum accumulates, the silica:alumina

ratio decreases. The Si0 ² /A1 ² 03 ratio decreases from 3:1 for albite to 1:1 for kaolinite. Other weathering reactions include oxidation, dissolution, hydration, reduction, and carbon- In the reaction a primary mineral, albite, was con- ation. An example of mineral weathering and the verted to kaolinite. The first-formed kaolinite formation of clay minerals on the surface of a particles are very small. They increase in size as piece of basalt is shown in Figure 10.2. additional ions attach or join the crystal, resulting in an gradual increase in particle size. The clay Summary Statement fraction of soils tends to be dominated by particles formed in this manner and particles like kao- 1. Mineral weathering is stimulated by an acid linite are called clay minerals. Clay minerals tend soil environment. to be resistant to further weathering. Therefore, as 2. Plant-essential nutrient ions are released to primary minerals, like albite, weather and disap- the soil solution and clay minerals (second- pear, clay minerals, like kaolinite, increase in ary) are formed. abundance. 3. Primary minerals disappear and secondary In the reaction, sodium was released as an ion, minerals, especially the clay minerals, accu- Na'. In addition, some of the silicon in the albite mulate. was incorporated into kaolinite and some into 4. Much of the silicon released in weathering is H4 SiO4 (silicic acid). The silicic acid tends to be removed from the soil by leaching, unless WEATHERING AND SOIL MINERALOGICAL COMPOSITION 151

prising about 47 percent, 28 percent, and 8 percent by weight, respectively (see Figure 10.1). Many soil minerals are silicate or aluminosilicate minerals. Oxygen is abundant on a weight basis, and because of the large size of the oxygen atom, oxygen occupies more than 90 percent of the volume of the earth's crust (and soil), as shown in Table 10.2. Note the generally small atomic size of the abundant cations listed in Table 10.2 as compared with the oxygen. In the minerals, the negative charge of oxygen is balanced by the positive charge of cations. It is helpful to think of soil minerals as being composed of Ping-Pong balls with much smaller balls (like marbles), in the there is limited precipitation and leaching is interstices. The Ping-Pong balls represent large ineffective. The silica:alumina ratio de- negatively charged oxygen atoms, and the small creases as soils become more weathered. balls represent the positively charged cations in the interstices. Silicon is a very abundant cation, which per- Weathering Rate and Crystal Structure forms a role in the mineral world similar to the Particle size, through its effect on specific surface, role carbon plays in the organic world. The silicon is an important factor affecting the weathering ion fits in an interstice formed by four oxygen rate of minerals. That is, a given mineral in the silt ions. The covalent bonding (electron sharing) be- fraction weathers faster than if the mineral is in tween oxygen and silicon forms a tetrahedron as the sand fraction. Chemical bonding within the shown in Figure 10.3. The silicon-oxygen tetrahe- mineral crystal, however, is the major factor af- dron is the basic unit in silicate minerals. The fecting weathering rate. valence of the silicon is +4 and that of the oxygen Oxygen, silicon, and aluminum are the three most abundant elements in the earth's crust, com-

TABLE 10.2 Size, Percent Volume in Earth's Crust, Coordination Number and Valence of the Most Abundant Elements in Soil Minerals 152 SOIL MINERALOGY

The coordination number is the number of ions suits in the H+ of the water replacing magnesium that can be packed around a central ion. Four and iron ions from the crystal face, rather than the oxygen ions can be packed around a silicon ion, silicon. The weathering of olivine can be viewed resulting in a coordination number of 4 (as shown as the separation of the Si-O tetrahedra with the Mg2+ ²+, in Figure 10.3). Note that larger cations have a release of the and Fe as illustrated in larger coordination number. Aluminum, iron, and Figure 10.4. The Si-O tetrahedra and the magne- magnesium ions are larger than silicon ions, and sium and iron ions encounter other ions in the aluminum, iron, and magnesium typically have a soil solution, where the ions can react and form coordination number of 6. Potassium has a coor- new minerals. The magnesium and iron ions can dination number of 8 and 12 (see Table 10.2). be absorbed by roots. Olivine weathers rapidly

Different silicate minerals are formed, depend- and soils withMg²+ a high content of olivine may have ing on the way the silicon-oxygen tetrahedra are so much in solution as to be detrimental to linked together and how the net charge of the some plants. Conversely, olivine is absent in tetrahedra is neutralized. In olivine, the silicon- many soils because of its low resistance to weath- oxygen (Si-0) tetrahedra are bonded together by ering. magnesium and/or iron. The crystal is neutral and The net charge of individual Si-O tetrahedra has the formula of MgFeSi0 4 . The ratio of magne- may be partly or entirely neutralized by the com- sium and iron, however, is highly variable, result- mon sharing of oxygen between adjacent tetrahe- ing in several olivine species. The model of oliv- dra. If adjacent Si-O tetrahedra share one oxygen, ine in Figure 10.4 shows that each magnesium single chains are formed as in pyroxene and au- ion, or iron ion, is surrounded by six oxygen ions; gite (see Figure 10.5). As shown in Figure 10.5, as three oxygen ions form each of the faces of two more sharing of oxygen by silicon occurs, miner- adjacent Si-0 tetrahedrons. Magnesium and iron als with double chains, sheets, and three- ions are larger than silicon and more oxygen ions dimensional structures are formed. Mica, and ka- are required to form an interstice large enough for olinite, have a sheet structure, and quartz has a them to occupy. The coordination number of three-dimensional structure. In the case of quartz magnesium and iron is 6. (Si0 ²) all of the charge is neutralized by the shar- The oxygen-silicon bonds are much stronger ing of all of the oxygen with silicon. The result is a than the magnesium and iron bonds with oxygen. mineral with strong chemical bonding within the Consequently, reaction of olivine with water re- crystal and great weathering resistance. As more

FIGURE 10.3 Models showing the tetrahedral (four-sided) arrangement of silicon and oxygen ions in silicate minerals. The silicon-oxygen tetrahedron on the left has the apical oxygen set off to one side to show the position of the silicon ion. WEATHERING AND SOIL MINERALOGICAL COMPOSITION 153

FIGURE 10.4 Models representing the weathering of olivine. Olivine is composed of silicon-oxygen tetrahedra held together by iron and/or magnesium. Every other tetrahedron is "inverted," as shown by the light-colored tetrahedra in the olivine model on the left. During weathering, the silicon- oxygen tetrahedra separate with the release of iron and magnesium. oxygen sharing occurs, the oxygen-silicon ratio of weathering and soil formation). The weathering (ratio of oxygen ions to silicon ions) of the min- stages are based on the representative minerals in eral decreases. This is associated with increased the fine silt and clay fractions. Table 10.3 contains weathering resistance. 13 weathering stages grouped into minimal, moderate, and intensive weathering stages. Weathering stage I represents soils that have Mineralogical Composition Versus been subjected to little, if any, weathering and Soil Age leaching, because the soils contain weatherable The difference in the weathering resistance of minerals such as gypsum, (CaSO 4 2H² 0), and ha- minerals has been used to develop a series of lite (NaCI). Each higher weathering stage is domi- weathering stages that parallel soil age (amount nated by minerals with greater weathering resis-

FIGURE 10.5 Models showing the common arrangements of silicon- oxygen tetrahedra in silicate minerals and their relation to weathering resistance. 154 SOIL MINERALOGY

TABLE 10.3 Representative Minerals and Soils Associated with Weathering Stages

tance, and in effect, more developed or older erately weathered and they are quite fertile soils. soils. Soils in which the fine silt and clay fractions Most of the world's corn and wheat are produced are dominated by minerals of the first five weath- on these soils. ering stages are considered to be minimally Soils of weathering stages 10 through 13 are weathered. These soils have sand and silt frac- considered to be intensively weathered. Minerals tions that are rich in primary minerals. The soils representative of the intensive weathering stages tend to occur where there has been a lack of time are secondary minerals with great weathering re- or water for weathering or where temperatures are sistance (see Table 10.3). These soils tend to oc- too low for effective weathering. Minimally weath- cur in warm and humid tropical regions where ered soils are generally fertile. there has been sufficient time to weather almost Soils dominated by minerals represented by all of the primary minerals. Many intensively moderate weathering are in stages 6 thru 9. These weathered soils have clay fractions dominated by soils tend to have a significant amount of both kaolinite, gibbsite (Al(OH) 3 ), and iron oxides, in- primary minerals (quartz and muscovite mica) cluding hematite. The world's oldest and most and secondary minerals (vermiculite and weathered soils are high in content of oxides of smectite). Many temperate region soils are mod- aluminum (gibbsite), iron (hematite), and/or tita- SOIL CLAY MINERALS 1RQ

environment of somewhat limited leaching. Crys- spaces between smectite particles contribute to tallization of volcanic ash products in seawater hydraulic impermeability. Houston Black Clay often produces montmorillonite. The expansion soils in Texas have a high smectite content and of layers permits easy diffusion of hydrated are characterized by contraction and develop- cations into the interlayer space and adsorption ment of wide cracks during the dry season and by on the planar surfaces. The dominant features of great expansion and closure of the cracks during montmorillonite are shown in Figure 10.12 and the wet season. some properties are given in Table 10.4. The individual clay layers in soils tend to occur as clusters that are much like a stack of playing Kaolinite cards. The more expansive the clay, the more Reference to the formation of kaolinite was made likely layers will separate, causing the clay to exist in regard to the weathering of albite feldspar. In as very small-sized particles. As a consequence, that case, kaolinite formation was by crystalliza- smectites occur as very small particles, which tion from solution. Another mechanism for kao- give this type of clay great capacity for swelling linite formation is the partial disintegration of 2:1 and shrinking with wetting and drying. The small clays. The stripping away of one Si-0 sheet, from

FIGURE 10.13 Model of kaolinite showing two offset layers. The upper layer surfaces are composed of hydroxyls (light-colored). These hydroxyl are bonded by the hydrogen of the hydroxyl to the oxygen of adjacent layers, producing a nonexpanding clay. Cation exchange capacity is low and originates at the edges due to deprotonation of exposed hydroxyls. A hydrated cation is adsorbed to such a cation exchange site. 160 SOIL MINERALOGY

a 2:1 layer, converts the 2:1 layer into a 1:1 layer, volcanoes and are deposited on land masses be- typical of kaolinite. This mode of formation is fore the atoms become organized into crystals. In favored by an acid weathering environment that is the humid tropics, amorphous minerals weather found in older soils or intensively weathered soils rapidly into an amorphous clay mineral called where silicon is being removed by leaching. allophane. Soils high in allophane have a gel-like The 1:1 structure has layers that have a plane of consistency when wet and lack the grittiness as- oxygens on one side and a plane of hydroxyls on sociated with sand particles. There is an enor- the other. Stacking of the layers results in a strong mous surface area and great reaction with, or attraction between oxygen of one layer and the adsorption of, organic matter, giving the organic hydrogen of hydroxyl (hydrogen bonding) of an matter protection against decomposition. Charac- adjacent layer, resulting in a nonexpanding clay. teristic features of soils high in allophane include Kaolinite particles tend to be large in size (many a high content of organic matter, high water- are silt sized), and to have great resistance to holding capacity, high and variable cation ex- further weathering. The tetrahedral and octahe- change capacity, and low load-bearing strength. dral sheets may retain some isomorphous substi- Slowly, allophane becomes more organized tution that can contribute to cation exchange ca- (more crystalline) and becomes halloysite-a pacity. This appears to be very minor and the mineral with structure and composition similar to cation exchange capacity is low, relative to the kaolinite. A weathering sequence from volcanic other clay minerals discussed thus far (see Table ash weathering is allophane-halloysite-kaolinite. 10.4). The cation exchange capacity occurs Soils high in allophane are important in the moun- mainly because of deprotonation of exposed hy- tain chain that extends from southern South droxyl at the edges. A model showing some America to Alaska and, also, in Hawaii. properties of kaolinite is given in Figure 10.13. The large size of kaolinite particles often gives soils high in kaolinite clay content good physical Oxidic Clays properties. Many of the kaolinite particles occur Kaolinite formation from 2:1 minerals is indicative in the coarse clay (0.0002 to 0.002 mm diameter) of a leaching regime that is very effective in re- and silt fractions. Smectite particles typically oc- moving silicon. Under such conditions, kaolinite cur in the fine clay (less than 0.0002 mm diame- may slowly weather, resulting in the decom- ter). Two Bt horizons may contain the same position of the layer sheets into their component amount of clay but, if the clay is kaolinite, the parts. The Si-O tetrahedra are transformed into horizon could be quite water permeable com- Si-OH tetrahedra, H4SiO4 , (silicic acid), that is pared with a Bt horizon containing smectite. soluble and removed by leaching. Aluminum-OH Smectite is a common clay of U.S. midwestern octahedra are released and slowly polymerize to and Great Plains soils, in United States, and a form an aluminum hydrous oxide (Al(OH) 3), significant number of the soils have very slowly or called gibbsite. Gibbsite particles tend to be platy impermeable Bt horizons. By contrast, kaolinite is and hexagonal in shape like the sheet shown in common in soils on the coastal plain of south- Figure 10.7. Many layers stack on top of each other eastern United States where soils are much older. and exist as clay particles in soils. Gibbsite is very Even so, impermeable claypan Bt horizons are not insoluble and resistant to further weathering and common on the southeastern coastal plain. is representative of weathering stage 11 (see Ta- ble 10.3). The mineral is common in intensively weathered soils that also contain abundant iron Allophane and Halloysite oxides, such as hematite (Fe ² 03) and goethite Noncrystalline (amorphous) minerals exist where (FeOOH). Only rare and unusual soils have a sig- glassy ash and cinders are thrown into the air by nificant amount of titanium oxide (anatase), ION EXCHANGE SYSTEMS OF SOIL CLAYS 16 1 which is representative of weathering stage 13. ogy of the clay fraction is dominated by oxidic Few parent materials contain enough titanium for clays, together with kaolinite. a significant amount of anatase to accumulate via 5. Volcanic activity produces amorphous ash weathering and soil formation. and cinders that weather to form amorphous The oxide clays are abundant in many red- allophane. Allophane slowly crystallizes to colored soils in the humid tropics. These soils are form kaolinite. typically acid and, in an acidic environment, 6. Over time, weathering produces changes in some of the exposed hydroxyls on particle sur- the mineralogical composition of soils, faces of oxidic clays adsorb a H + (protonate) which is reflected in the soil's physical and and form sites of positive charge (Al-OH + H + = chemical properties. The cation exchange ca- Al-OH² +). The kaolinite clay has a net negative pacity, for example, increases to a peak with charge, which promotes attraction of the kaolinite weathering in soils dominated by 2:1 clays and oxidic clays and formation of extremely sta- (moderately weathered), which is followed ble aggregates or peds. The peds, which are by a very low cation exchange capacity in dense and frequently the size of sand grains, are soils dominated by oxidic clays (intensively very resistant to deformation and give the soil weathered). physical characteristics similar to soils dominated by quartz sand; that is, soils with an abundance of oxidic clays have high water-infiltration I ON EXCHANGE SYSTEMS OF rates and resist erosion. The soil can be plowed SOIL CLAYS shortly after a rain. A small amount of available water is retained at field capacity. The water is Some soils are dominated by layer silicate clays, held at low tension or high potential, and the some by oxidic clays, and some soils have both water is easily and rapidly absorbed by roots. kinds of clay in abundance. This results in three Short periods of water stress for crops may occur unique ion-exchange systems in mineral soils: even though the soils are in the tropical rain forest (1) layer silicate, (2) oxide, and (3) oxide-coated, zone. layer silicate. The type and kind of ion exchange have important consequences for plant growth and soil management. Summary Statement 1. There is a kinship amoung the crystalline alu- Layer Silicate System minum silicate clay minerals because all con- Layer silicate system soils are overwhelmingly sist of Si-0 tetrahedra sheets and Al-OH octa- negatively charged with high cation exchange ca- hedra sheets. pacity (CEC). The cation exchange capacity origi- The major differences in the aluminum sili- 2. nates mainly from isomorphous substitution. cate clays result from the ratio of tetrahedra and octahedra sheets and the nature and These negatively charged sites constitute permanent charge that is not affected by pH. The charge amount of isomorphous substitution. A summary of the major features of these clays is may be modified, however, to a small extent by minor or thin coatings of oxidic clays on the parti- given in Table 10.4. cle surfaces. Some cation exchange capacity orig- 3. The 2:1 clays are transformed into 1:1 clay by inates from the dissociation of hydrogen (depro- the loss of a Si-O tetrahedral sheet. This is one tonation) from edge AI-OH groups that is favored formation mode of kaolinite. by high pH (high OH concentration): 4. Near the end of the weathering sequence, gibbsite formation becomes important. In the [clay edge]AI-OH + OH - - oldest and most weathered soils, the mineral- = [clay edgelAl-0 + H ² O REFERENCES 163 diate between the other two systems. Many of the Ultimately, soils become "weathered out" and red-colored tropical soils have an ion exchange have almost no capacity to support plants. Such system that is greatly affected by both silicate and soils may be mined for ore. Clay edge Al-OH oxidic clay. protonate in acid soils and form sites of anion exchange capacity and deprotonate in alkaline soils and form sites of cation exchange ca- SUMMARY pacity. In an acid soil environment, weathering of minerals is stimulated. Minerals weather at different REFERENCES rates, depending on their resistance to weath- Birkeland, P. W. 1974. Pedology, Weathering, and Geo- ering. morphological Research. Oxford, New York. In general, primary minerals weather and sec- Bohn. H. L., B. L. McNeal, and G. A. O'Connor. 1985. ondary minerals are formed. Over time, the miner- Soil Chemistry, 2nd ed. John Wiley, New York. alogical composition of the soil changes and, Clarke, F. W. 1924. "Data of Geochemistry." U.S. Geo. consequently, soil properties change. Survey Bull. 770. Washington, D.C. Weathering releases ions to the soil solution. Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. John Many of these ions are plant nutrients. Some ions Wiley, New York. precipitate to form secondary minerals and some Jackson, M. L. and G. D. Sherman. 1953. "Chemical ions are leached from the soil. Weathering of Minerals in Soils." Advances in Minimally and moderately weathered soils tend Agronomy. 5:219-318. to be rich in primary minerals and to be fertile Jenny, H. 1980. The Soil Resource. Springer-Verlag, with high cation exchange capacity. Intensively New York. Soil Sci. Soc. Am. 1977. Minerals in the Soil Environ- weathered soils are depleted of primary minerals. ments. Soil Sci. Soc. Am. Madison, Wis. Few nutrient ions are released and soils are infer- Uehara, G. and G. Gillman. 1981. Mineralogy, Chem- tile. The clay minerals in intensively weathered istry, and Physics of Tropical Soils with Variable soils have low cation exchange capacity. Charge. Westview, Boulder. Nutrient ions adsorbed to the surfaces of clay White, R. E. 1987. Introduction to the Principles and particles represent a reservoir of nutrients that is Practice of Soil Science. 2nd ed. Blackwell Sci. Pub. exchangeable and available to plants. London. CHAPTER 11

SOIL CHEMISTRY

A total chemical analysis of a soil may reveal little age chemical composition of igneous rocks is that is useful in predicting the ability of the soil to similar to the mineralogical composition of many supply plant nutrients. Instead, it is usually a rela- minimally and moderately weathered soils. Such tively small part of the total amount of an element soils contain much quartz, feldspar, and mica in that is available for plant growth. For these rea- the sand and silt fractions, 2:1 layer silicate clays sons, the discussion of soil chemistry focuses on in the clay fraction, and much of the negative ion exchange reactions, solubilities, and mineral charge of the clays neutralized by the adsorption and biochemical transformations. These reac- of calcium, magnesium, sodium, and potassium tions are illustrated in Figure 11.1. ions. When soils weather and the mineralogical composition changes over time, there is a corresponding change in chemical composition. Dur- CHEMICAL COMPOSITION OF SOILS ing soil formation, there is a preferential loss of silicon relative to aluminum and iron. This is re- The average chemical composition of igneous flected in the chemical analysis of the intensively rocks is given in Table 11.1. On an oxide basis, weathered Columbiana clay soil in Costa Rica silicon and aluminum are first and second in (see Table 11.1). The soil contains only 26 per- abundance. This reflects the dominance of sili- cent silica, and the sum of iron and alumumium is cate and aluminosilicate minerals in igneous 69 percent (on an oxide basis). The silica: rocks. Iron is next in abundance in igneous rocks. alumina ratio decreased from 3.75 to 0.53 as a Next most abundant, are four cations that are result of weathering and soil formation. The important in balancing the charge of silicon- chemical composition of the Columbiana soil re- oxygen tetrahedra in minerals-calcium, magne- flects a high oxidic clay content. Release and loss sium, sodium, and potassium. The remaining 1 or by leaching of calcium, magnesium, sodium, and 2 percent includes the other elements. The aver- potassium were more rapid than silica, and this is

164 ION EXCHANGE 165

FIGURE 11.1 Major chemical processes and reactions in soils. Weathering of minerals and decomposition of organic matter supply ions to the ion pool of the soil solution. Ions are lost from the pool by plant uptake and leaching. Some ions precipitate and some ions form secondary minerals, especially clay minerals, and are subject to dissolution. Ions also sorb onto and desorb from cation and anion exchange sites. reflected in the low content of these four cations tively and postively charged surfaces, respec- in the intensively weathered soil relative to igne- tively. Such ions are readily replaced or ex- ous rocks. changed by other ions in the soil solution of similar charge, and thus, are described by the term, ion exchange. Of the two, cation exchange of greater abundance in soils than anion ex- ION EXCHANGE is change. Ion exchange is of great importance in soils (see Figure 11.1). Ion exchange involves cations and anions that are adsorbed from solution onto nega- Nature of Cation Echange Cation exchange is the interchange between a cation in solution and another cation on the sur- TABLE 11.1 Chemical Composition of Average face of any negatively charged material, such as Igneous Rocks and an Intensively Weathered Soil clay colloid or organic colloid. The negative charge or cation exchange capacity of most soils is dominated by the secondary clay minerals and organic matter (especially the stable or humus portion of SOM). Therefore, cation exchange reactions in soils occur mainly near the surface of clay and humus particles, called micelles. Each micelle may have thousands of negative charges that are neutralized by the adsorbed or exchangeable cations. Assume for purposes of illustration that X - represents a negatively charged ex- changer that has adsorbed a sodium ion (Na), producing NaX. When placed in a solution containing KCI, the following cation exchange reaction occurs: Adapted from Bohn, McNeal, and O'Connor, 1985. 166 SOIL CHEMISTRY

In the reaction, K+ in solution replaced or exchanged for adsorbed (exchangeable) Na', resulting in putting the adsorbed Na' in solution and leaving K + adsorbed as KX. Cations are adsorbed and exchanged on a chemically equivalent basis, that is, one K+ replaces one Na', and two K+t+ are required to replace or exchange for one Ca . The negatively charged micellar surfaces form a boundary along which the negative charge is localized. The cations concentrate near this boundary and neutralize the negative charge of the micelle. The exchangeable cations are hydrated from clay surface and drag along the hydration water molecules as Distance FIGURE 11.2 Distribution of cations and anions in the vicinity of a negatively charged surface. they constantly move and oscillate around negatively charged sites. The concentration of cations is greatest near the micellar surfaces, where the negative charge is the strongest. The charge Cation Exchange Capacity of Soils strength decreases rapidly with increasing dis- The cation exchange capacity of soils (CEC) is tance away from the micelle, and this is associ- defined as the sum of positive (+) charges of the ated with a reduction in cation concentration adsorbed cations that a soil can adsorb at a specific pH. away from the micelle. Conversely, the negatively Each adsorbed K+ contributes2+ one + charged micellar surface repels anions. This recharge, and each adsorbed Ca contributes sults in a decreasing concentration of cations and two + charges to the CEC. The CEC is the sum of an increasing concentration of anions with dis- the + charges of all of the adsorbed cations. tance away from the micellar surface. At some (Conversely, the CEC is equivalent to the sum of distance from the micellar surface, the concentra- the - charges of the cation exchange sites.) tion of cations and anions is equal. These features The CEC is commonly expressed as centimoles are illustrated in Figure 11.2. of positive charge per kilogram [cmol(+)/kg],

An equilibrium tends to be established between also written as cmol/kg, of oven dry soil. A mole23 the number of cations adsorbed and the number of positive charge (+) is equal to 6.02 x 10 of cations in solution. The number of cations in charges. For each cmol of CEC per kg there are solution is much smaller than the number ad- 6.02 x 1021 + charges on the adsorbed cations. sorbed (generally 1 percent or less) unless the For many years the CEC of soils was expressed content of soluble salts is high. Roots absorb as milliequivalents of cations adsorbed per 100 cations from the soil solution and upset the equi- grams of oven dry soil (meq/ 100 g). Many agricul- librium. The uptake of a cation is accompanied by tural publications still use meq/100 g, which the excretion of H+ from the root and this restores means that many persons need to be familiar with the charge equilibrium in both the plant and soil. both systems. The CEC values are the same in Excretion of hydrogen ions tends to increase the both systems; that is, a CEC of 10 cmol/kg is equal acidity in the rhizosphere and the H + may be to a CEC of 10 meq/100 g adsorbed on micelles. The H + is weakly adsorbed The CEC of a soil is equal to the CEC of both the onto clay (ionic bonding); however, it is strongly mineral and organic fractions. The major source adsorbed to carboxyl groups of humus (covalent of CEC in the mineral fraction comes from the bonding). clay. The negative charge of organic matter is due ION EXCHANGE 167 mainly to dissociation of H+ (deprotonation) from the -OH of carboxyl and phenolic groups. The CEC of soil organic matter (SOM) ranges from 100 to 400 cmol/kg, depending on degree of decomposition. There is an increase in the groups contributing to the CEC as organic matter becomes more decomposed or more humified. As mentioned in the previous chapter, the clays have great variation in CEC, ranging from less than 10 cmol/kg for oxidic clay, to a 100 cmol/kg or more for some 2:1 clay. The sand and silt fractions contribute little to the CEC of soils. As a consequence, the CEC of soils is affected mainly by the amount and type of clay and the amount and degree of decomposition of the organic matter. In the A horizons of mineral soils, the organic matter and clay frequently make similar contributions to the CEC. In subsoils, such as Bt horizons where clay has accumulated, the clay commonly contributes more to the CEC than the organic matter. The accumulation of humus in Bhs or Bh horizons contributes much CEC to these subsoil horizons. FIGURE 11.3 The cation exchange capacities in a sand-textured soil at the soils natural pH and determined at pH 7.0 and 8.2. The increases in cation exchange capacity with increasing pH are Cation Echange Capacit versus closely related to the content of organic carbon Soil pH (organic matter) in this soil which has a Bh horizon The definition of CEC specifies that the CEC ap- and contains very little clay. (Adapted from Holzhey, plies to a specific pH because the CEC is pH Daniels, and Gamble, 1975). dependent. To make valid comparisons of CEC between soils and various materials, it is necessary to make the determination of CEC at a common pH. The CEC is positively correlated with pH; important in the management of intensively therefore, acid soils have a CEC less than the weathered and acid soils in tropical regions be- maximum potential CEC. cause of the generally low CEC and the highly Most of the CEC of 2:1 clays is permanent or pH-dependent nature of the CEC. For minimally non-pH dependent. By contrast, the CEC of SOM is and moderately weathered mineral soils, where entirely pH-dependent. Soils high in organic mat- much of the CEC originates from the permanent ter content, kaolinite (1:1 clay), and oxidic clays charge of 2:1 clays, changes in CEC resulting from have relatively large changes in CEC with changes changes in soil pH have much less importance. in pH owing to the predominance of pH- For plants, the current soil pH is the relevant pH. dependent charge. The changes in the CEC with Values given for the CEC are for soils at their changes in pH of the horizons in a very sandy soil natural, or current pH, unless otherwise noted. that has a Bh horizon is shown in Figure 11.3. The CEC at the soils' current pH is called the Changes in CEC with changes in soil pH are effective cation exchange capacity (ECEC). 168 SOIL CHEMISTRY

Kinds and Amounts of Exchangeable Cations The major sources of cations are mineral weathering, mineralization of organic matter, and soil amendments, particularly lime and fertilizers. The amounts and kinds of cations adsorbed are the result of the interaction of the concentration of cations in solution and the energy of adsorption of the cations for the exchange surface. The cations compete for adsorption. As the concentration of a cation in the soil solution increases, there is an increased chance for adsorption. The more strongly a cation is attracted to the exchange surface (energy of adsorption); the greater is the chance for adsorption. The energy of adsorption of specific ions is related to valence and degree of hydration. The cules are attracted to ions, the greater is the num- energy of adsorption of divalent cations is about ber of hydrated molecules and the hydrated ra- twice that of monovalent cations. For cations of dius of the ions. equal valence, the cation with the smallest hy- The differences in the hydrated radius and va- drated radius is most strongly adsorbed because lence of Ca t+ and Na' are illustrated in Figure it can move closer to the site of charge. The dehy- 11.4. Calcium is adsorbed more strongly than so- drated and hydrated radi of four monovalent dium because: (1) it is divalent, and (2) it has the cations are given in Table 11.2. Rubidium (Rb) is smallest hydrated radius. As a result of the small the most strongly adsorbed, whereas lithium (Li) energy of adsorption of sodium, it is more likely to is the most weakly adsorbed. Note that the hy- exist in the soil solution and be removed from the drated radius is inversely related to the nonhy- soil by leaching. As a result of the strong energy of drated radius. This is because the water mole- adsorption, calcium is typically more abundant as cules can move closer to the charge that is located an exchangeable cation than are magnesium, po- at the center of a small ion and, therefore, the tassium, or sodium. The energy of adsorption se- water molecules are more strongly attracted to quence is: Ca >Mg > K > Na. The cmol/kg smaller ions. The more strongly that water mole- (meq/ 100 g) of these four cations for a moderately weathered soil is given in Table 11.3. Note that their abundance follows the energy of adsorption sequence and the dominance of calcium. TABLE 11.2 Ionic Radii and Exchange Efficiency of The symbol, X, has already been used in a Several Monovalent Ions cation exchange reaction, as in NaX. In NaX, a Na' is adsorbed onto a cation exchange site. By transposing the X to form XNa, exchangeable sodium is formed. XNa is an abbreviation for exchangeable Na. Pronounce XNa, as "X Na," or as exchangeable Na, and write it as XNa. Ex- changeable calcium is XCa. This nomenclature will be used in reference to the exchangeable cations. ION EXCHANGE 169

TABLE 11.3 Exchangeable Calcium, Magnesium, data in Table 11.3. A plow layer would contain Potassium and Sodium in Tama Silt Loam enough XCa to last a high-yielding corn crop 112 years, the XMg would last for 27 years, whereas the XK would provide only a 2-year supply. When the exchangeable cations in the underlying horizons are considered, it can be seen that the exchangeable cations are an important supply of available nutrients. Small, but important, quantities of other cations, including iron, manganese, copper, and zinc also exist in the soil as exchangeable cations.

Adapted from Soil Survey Investigations Report, No. 3, I owa, SCS, USDA, 1966. Anion Exchange Anion exchange sites arise from the protonation of hydroxyls on the edges of silicate clays and on Exchangeable Cations as a Source of the surfaces of oxidic clays. The anion exchange Plant Nutrients capacity is inversely related to soil pH and is, Of the four cations referred to in Table 11.3, three perhaps, of greatest importance in acid soils dom- - are essential for plants. The sodium may be ab- inated by oxidic clays. Nitrate (N03 ) is very sorbed by plants, may substitute for some po- weakly adsorbed in soils and, as a consequence, tassium, but sodium is not considered an essen- remains in the soil solution where it is very sus- tial element. It is, however, needed by animals, ceptible to leaching and removal from soils. and its uptake by plants and its presence in food is Anion exchange reactions with phosphate may important. It is paradoxical that, in regard to XCa result in such strong adsorption as to render the and XK, the XCa is much more abundant in soils phosphate unavailable to plants. Uptake of anions than is XK. On the other hand, plants generally by roots is accompanied by the excretion of OH - - absorb much more potassium than calcium. The or HC03 . result is that calcium is rarely deficient in soils for In general, plants absorb as many anions as plant growth; by contrast, potassium frequently cations. The availability to plants of the anions limits crop yields in the humid regions and is a nitrate, phosphate, and sulfate is related to miner- common element in fertilizers. This is reflected in alization from organic matter, as well as anion the data of Table 11.4, which are based on the exchange.

TABLE 11.4 Amounts of Exchangeable Calcium, Magnesium and Potassium in 2 Million Pounds in the Ap horizon of Tama Silt Loam and Adequacy for Plant Growth 17 0 SOIL CHEMISTRY

SOIL pH herit these properties from the parent material. Throughout the world there are parent materials The pH is the negative logarithm of the hydrogen and soils that contain several percent or more of ion activity. The pH of a soil is one of the most important properties involved in plant growth. calcium carbonate; they are calcareous. When There are many soil pH relationships, including calcareous soil is treated with dilute HCI, carbon those of ion exchange capacity and nutrient avail- dioxide gas is produced, and the soil is said to effeece. ability. For example, iron compounds decrease in solubility with increasing pH, resulting in many instances where a high soil pH (soil alkalinity) Carbonate Hydrolysis causes iron deficiency for plant growth. The hydrolysis of calcium carbonate produces OH - , which contributes to alkalinity in soils: Determination of Soil pH Soil pH is commonly determined by: (1) mixing one part of soil with two parts of distilled water or Calcium carbonate is only slightly soluble, and neutral salt solution, (2) occasional mixing over a this reaction can produce a soil pH as high as 8.3, water. to period of 30 minutes to allow soil and assuming equilibrium with atmospheric carbon approach an equilibrium condition, and, (3) mea- dioxide. In calcareous soil, carbonate hydrolysis suring the pH of the soil-water suspension using a controls soil pH. When a soil contains Na 2 CO 3 , pH meter. A rapid method using pH indicator dye the pH may be as high as 10 or more, which is is illustrated in Figure 11.5. The common range of caused by the greater solubility of Na 2 CO 3 soil pH is 4 to 10. and greater production of OH - by hydrolysis in a similar manner. Sodium-affected soils with Sources of Alkalinit 15 percent or more of the CEC saturated with Parent materials have a wide ranging mineral- Na' are called sodic and are also highly alka- ogical composition and pH, and young soils in- line.

FIGURE 11.5 To determine soil pH with color indicator dye solution: (1) Place a small amount of soil in the folded crease of a strip of wax paper; (2) add 1 or 2 drops more indicator than the soil can absorb, tilt up and down to allow indicator to equilibrate with soil, and (3) separate excess dye from soil with tip of a pencil and match dye color with the color chart.

SOIL pH 171

Mineral Weathering Some rocks and minerals Sources of Acidit weather to produce an acidic effect. The weather- Three important processes, or reactions, contrib- + ing of many primary minerals, however, contribute to a more or less continuous addition of H to utes to alkalinity. This is the result of the con- soils, and these processes tend to make soils sumption of H + and the production of OH - . For acid. In all soils respiration by roots and other soil example, the hydrolysis of anorthite, (calcium organisms produces carbon dioxide that reacts feldspar), produces a moderately strong base: with water to form carbonic acid (H2CO3). This is a weak acid, which contributes H+ to the soil solution. Second, acidity is produced when organic matter is mineralized, because organic acids are formed and the mineralized nitrogen and sulfur are oxidized to nitric and sulfuric acid, In the reaction, an aluminosilicate clay mineral respectively. Third, natural or normal precipi- was formed together with the moderately strong tation reacts with carbon dioxide of the atmo- base, calcium hydroxide. The net effect is basic. sphere and the carbonic acid formed gives natural The generalized weathering reaction, in which M precipitation a pH of about 5.6. The results of represents metal ions such as calcium, magne- these reactions add acidity continuously to soils. sium, potassium, or sodium, is In desert regions there is little precipitation and, thus, these reactions contribute little acidic input to soils. As precipitation increases there is an increase in all three of the previously mentioned processes. A larger amount of precipitation As long as a soil system remains calcareous, contains more acidity. Greater precipitation re- carbonate hydrolysis dominates the system and sults in more plant growth, causing more respira- maintains a pH that ranges from 7.5 to 8.3 or tion and organic matter mineralization. Many more. Mineral weathering is minor under these studies have shown a relationship between an- conditions, and the mineralogy and pH of the nual precipitation, the leaching of carbonates, soils change little, if at all, with time. The develop- and soil pH, as given in Figure 11.6. ment of soil acidity requires the removal of the In soils with a pH of 7, there is a balance in the carbonates by leaching. When acidity develops, processes or reactions that produce alkalinity and the weathering of primary minerals is greatly in- acidity. On the central U.S. Great Plains this oc- creased, causing an increase in the release of curs with about 26 inches (65 cm) of annual pre- cations-calcium, magnesium, potassium and cipitation (near the Iowa-Nebraska border). The sodium. Leaching of these cations in humid re- dominant upland soils have developed under a gions, however, eventually results in the develop- grass vegetation and are quite fertile for agricul- ment of soil acidity. tural crops. Calcium and magnesium are alkaline earth metals or cations, and potassium and sodium are alkali metals. These cations are called basic Development and Properties of cations because soils tend to be alkaline (basic) Acid Soils or neutral when the CEC is entirely saturated with In a calcareous soil, OH - from carbonate hydroly- them. Leaching of the basic cations (Ca, Mg, K, sis reacts with any H + that is produced to form and Na) results in their replacement by Al" and water; thus, calcareous soils remain alkaline in H+, and soils then become acid. The cations A13 + spite of the near constant production or addition and H+ are called the acidic caion. of H + . As long as soils remain calcareous, they 172 SOIL CHEMISTRY

FIGURE 11.6 General relationship of upland soils between annual precipitation, depth of leaching of carbonates, and pH of the surface soil in the central United States. (Adapted from Jenny, 1941.)

remain alkaline. Development of neutrality, and leached soils, all of the horizons are acid. By eventually acidity, is dependent on the removal of contrast, many moderately weathered and the carbonates. This requires sufficient precipi- leached soils have acid A and B horizons and tation to cause effective leaching. This amount of nuetral or alkaline (many being calcareous) C precipitation also results in considerable acidic horizons. input. The weathering of primary minerals and the release of basic cations tend to repress the acidifying effects of respiration, precipitation, and min- Role of Aluminum After leaching has removed eralization of organic matter. Any H + in the soil the carbonates, a progressive loss of XCa, XMg, solution neutralizes the OH - produced by min- XK, and XNa occurs as leaching continues. For eral weathering and shifts the weathering reac- example, the removal of an exchangeable potassium ion, K+, is accompanied by the removal tions to the right, or encourages further weather- + ing of primary minerals and formation of OH - . of an acid anion, like nitrate. The H associated Despite the neutralizing effect of mineral weather- with the nitrate remains behind and competes for ing, soils in humid regions become increasingly adsorption onto the exchange position vacated by acid with time as leaching continues. the potassium. Adsorption of H+ at the edge of The effects of the acidifying processes are pro- silicate clay minerals makes aluminum unstable, gressive; the effects producing acid A horizons and it exits from the edge of the clay lattice. Subsequent hydrolysis of the released A1 3 + pro- first, then acid B horizons, and, lastly, acid C + horizons. Many minimally weathered and leached duces H : soils have a neutral or alkaline reaction in all horizons and in intensively weathered and

SOIL pH 173

Hydroxy-aluminum from the preceding reaction exchangeable cation (XAI). The aluminum ion + hydrolyzes to produce additional H + : has a valence of 3 , and it is adsorbed much more strongly than divalent and monovalent cations. The amount of XAI increases over time as the soil becomes more leached and more acid. The in- Consequently, acidity in soil stimulates the devel- crease in XAL is accompanied by a decrease in opment of additional acidity through aluminum XCa, XMg, XK, and XNa. hydrolysis, and aluminum hydrolysis becomes a very important source of H+ when soils become acid. Moderatel versus Intensivel Weathered The hydroxy-aluminum (hydroxy-Al) formed in Soils As long as soils remain only moderately the preceding reactions consists of AI-0H octahe- weathered, the weathering of primary minerals dra that polymerize and produce large cations and the release of calcium, magnesium, po- with a high valence. These large cations are very tassium, and sodium contribute to the mainte- strongly adsorbed onto cation exchange sites be- nance of a good supply of these elements. These cause of their high valence. In fact, the adsorption elements, or basic cations, occupy most of the is so strong that they are not exchangeable in the cation exchange sites, and there is little ex- sense that XCa and XNa are exchangeable. Sites changeable aluminum. As moderately weathered that adsorb hydroxy-aluminum cease to func- soils evolve into intensively weathered soil, tion for cation exchange, and the adsorption of however, a considerable change in mineralogical hydroxy-aluminum reduces the CEC. This is one composition occurs. The high CEC 2:1 clays dis- more reason for the soil's pH-dependent cation appear, and low CEC clays, such as kaolinite and exchange capacity. The adsorption of hydroxy- oxidic clay, accumulate. There is a large decrease aluminum in the interlayer space of expanding in the CEC, and soils have a small capacity for clays renders them nonexpandable. The ad- adsorption of nutrient cations. In addition, with sorption of large hydroxy-aluminum cations in the the disappearance of most of the primary miner- interlayer space of 2:1 clay is illustrated in Figure als, there is a very limited release of calcium, 11.7. magnesium, potassium, and sodium by weather- 3 When soil pH declines below 5.5, A1 + begins ing. The aluminum saturation of the CEC ranges to occupy cation exchange sites and exist as an from about 0 percent aluminum saturation at pH 5.5 to nearly 100 percent aluminum (XAl) saturation at pH 4. The saturation of the CEC with basic FIGURE 11.7 Drawing of two layers of a 2:1 cations (calcium, magnesium, potassium, and expanding clay with hydroxy-Al adsorbed in the potassium) decreases from nearly 100 percent at interlayer space. This adsorption is very strong and pH about 5.5 or 6 to about 0 percent at pH 4.0, reduces the cation exchange capacity. The mutal causing a great decline in the amount of ex- attraction of the two clay layers to the positively changeable nutrient cations. In essence, intensive charged hydroxy-Al causes the clay to become a nonexpanding, hydroxy-interlayered clay. weathering results in acid soils that are naturally infertile. The data in Table 11.5 are for three intensively weathered soils on the coastal plain of the southeastern United States. The pH is below 5, and the CEC averages only 2.68 cmol/kg (2.68 meq/ 100 g). These soils are characterized by very low fertility as suggested by the small amount of XCa 174 SOIL CHEMISTRY

TABLE 11.5 The pH, Exchangeable Cations (KCI Extractable), Cation Exchange Capacity, and Aluminum Saturation in Three Intensively Weathered Soils

and XMg (XK and XNa are also low). The average acid and low pH make reclamation and revegeta- aluminum saturation of the cation exchange cation of such materials difficult. pacity (cmol of XAI divided by the CEC and times 100) is 88 percent. When the aluminum saturation of soils exceeds 60 percent, there is frequently Acid Rain Effects The water in precipitation enough aluminum (A1 3+ ) in solution to be toxic reacts with carbon dioxide in the atmosphere to to plant roots. The relationship between the form carbonic acid. This causes natural, or ordi- amounts of exchangeable aluminum and calcium nary, precipitation to be acidic and have a pH of and the growth of corn roots is shown in Figure about 5.6. Industrial gases contain nitrogen and 11.8. sulfur that result in the formation of acids and Whereas calcium and magnesium deficiences cause rainfall to have a pH less than 5.6. Such rain for plants are rather uncommon on acid soils that is called acid rain. Studies have shown that are moderately weathered, low XMg and XCa the amount of acidity contributed to soils by acid commonly result in plant deficiences on acid and rain is usually much less than that normally intensively weathered soils. A comparison of the contributed by respiration, organic matter data in Table 11.5 with the data in Table 11.3 mineralization, and the acidity in normal rain- shows these large differences in exchangeable fall. cations. In soils that are underlain by calcareous layers, any acid water that percolates through the soil will be neutralized and will not contribute to the Role of Strong Acids A soil pH below about 3.5 acidity of groundwater or nearby lakes. On the and down to 2 is indicative of strong acid in the Laurentian Shield in northeastern Canada, soils soil. Pyrite (FeS 2) occurs in some soils and the generally are underlain by acid parent materials sulfur is oxidized to sulfuric acid. Soils containing and rocks. Here, additions of acidity from acid strong acid may produce a toxicity of both alumi- rain to acid soils contribute to the acidification of num and iron; the soil may be too acid for plant groundwater and lakes. growth. These soils are called acid sulfate soils or cat clays, and they tend to form in mangrove swamps along saltwater beaches (see Figure Soil Buffer Capacit 11.9). Sometimes the tailings from mining opera- The buffer capacity is the ability of ions associ- tions contain pyrite, and the formation of sulfuric ated with the solid phase to buffer changes in ion SOIL pH 175

FIGURE 11.8 On left, poor corn root growth in a Hawaiian soil associated with low exchangeable calcium and high exchangeable aluminum. On right, good corn root growth associated with low exchangeable aluminum and high exchangeable calcium. (Drawn from Green, R. E., Fox, R. L., and D. D. F. Williams, 1965.)

FIGURE 11.9 Mangrove swamps are one of the concentration in the solution phase. In acid soils, typical environments where acid sulfate soils buffering refers to the ability of the XAl, XH, and develop. hydroxy-aluminum to maintain a certain concentration of H+ in solution. The amount of H+ in the soil solution of a soil with a pH of 6.0, for example, is extremely small compared with the non- dissociated H+ adsorbed and the amount of aluminum that can hydrolyze to produce H + . Neu- tralization of the active or solution H + results in rapid replacement of H+ from the relatively large amount of H + associated with the solid phase. Thus, the soil exhibits great resistance to undergo a pH change. 176 SOIL CHEMISTRY

Summar Statement mainly by hydroxy-aluminum hydrolysis, and With a long period of time, soil evolution in humid in the range 5.5 to about 4 the H+ for the soil regions causes changes in both the mineralogical solution comes mainly from exchangeable and chemical properties and changes in soil pH. aluminum hydrolysis. Soils high in organic Minimally weathered soils tend to be alkaline or matter may receive a significant amount of slightly acid and fertile. By contrast, intensively H+ from exchangeable hydrogen. weathered soils are very acid and infertile. A sum- 5. Intensively weathered soils in the advanced mary of the major processes and changes that stages of weathering have: (1) low CEC, produce various ranges in soil pH and accom- (2) high saturation of the cation exchange panying properties is as follows. capacity with aluminum, and (3) very small amounts of exchangeable calcium, magne- 1. In the pH range 7.5 to 8.3 or above, soil pH is sium, potassium, and sodium. Intensively controlled mainly by carbonate hydrolysis. weathered soils tend to produce aluminum 2. If a soil is calcareous, removal of carbonates toxicity and deficiencies of magnesium and by leaching is required before a neutral or calcium for plant growth. acid soil can develop. 6. In the most acid soils, pH < 4, free strong 3. Soils naturally at or near neutral tend to occur acid exists. in regions of limited precipitation where there is a balance between the production of H + and OH - . The acidic inputs from biological SIGNIFICANCE OF SOIL pH respiration, organic matter mineralization, and precipitation are balanced by the basic Studies have shown that the actual concentrations inputs of mineral weathering. of H + or OH - are not very important, except un- 4. In humid regions, soils tend to become acid der the most extreme circumstance. The associ- over time as basic cations decrease and ated chemical or biological environment of a cer- acidic cations increase. In the range of about tain pH is the most important factor. Some soil 7 to 5.5, H+ for the soil solution is supplied organisms have a rather limited tolerance to varia-

FIGURE 11.10. Red pine was planted along the edges of this field to serve as a windbreak. The red pine in the foreground and background failed to survive because these areas were part of an old limestone road and the soil was alkaline. White cedar was later planted and is growing very well on the old road. SIGNIFICANCE OF SOIL pH 177 tions in pH, but other organisms can tolerate a duced by the presence of calcium carbonate that wide pH range. The effects of limestone gravel represses the dissolution of calcium phosphates. from an old roadway on the growth of red pine Maximum phosphorus availablity is in the range and white cedar trees is shown in Figure 11.10. 7.5 to 6.5. Below pH 6.5, increasing acidity is associated with increasing iron and aluminum in solution and the formation of relatively insoluble Nutrient Availabilit and pH iron and aluminum phosphates. Perhaps the greatest general influence of pH on Note that potassium, calcium, and magnesium plant growth is its effect on the availability of are widely available in alkaline soils. As soil nutrients for plants, as shown in Figure 11.11. acidity increases, these nutrients show less avail- Nitrogen availability is maximum between pH 6 ability as a result of the decreasing CEC and and 8, because this is the most favorable range for decreased amounts of exchangeable nutrient the soil microbes that mineralize the nitrogen in cations: decreased amounts of XCa, XMg, and XK. organic matter and those organisms that fix nitro- Iron and manganese availability increase with gen symbiotically. High phosphorus availability at increasing acidity because of their increased sol- high pH-above 8.5-is due to sodium phos- ubility. These two nutrients are frequently defi- phates that have high solubility. In calcareous cient in plants growing in alkaline soils because soil, pH 7.5 to 8.3, phosphorus availability is re- of the insolubility of their compounds (see Figure

FIGURE 11.11 General relationship between soil pH and availability of plant nutrients in minimally and moderately weathered soils; the wider the bar, the more availability. 178 SOIL CHEMISTRY

11.11). Boron, copper, and zinc are leachable and earthworms. Since earthworms are the primary can be deficient in leached, acid soils. Con- food of moles, the moles restricted their activity versely, they can become insoluble (fixed) and only to the soil under the marking lines. unavailable in alkaline soils. In acid soils, molybdenum is commonly deficient owing to its reaction with iron to form an insoluble compound. Toxicities in Acid Soils The relationships shown in Figure 11.11 are for Figure 11.11 shows that iron and manganese are minimally and moderately weathered soils. For most available in highly acid soils. Manganese plant nutrients as a whole, good overall nutrient toxicity may occur when soil pH is about 4.5 or availability occurs near pH 6.5. In intensively less. High levels of exchangeable aluminum in weathered soils, some nutrients such as boron many acid subsoils of the southeastern United and zinc may be in very short supply and become States cause high levels of solution aluminum and deficient at lower pH than for minimally and mod- restrict root growth. Plants, and even varieties of erately weathered soils. As a consequence, a de- the same species, exhibit differences in tolerance sirable pH is more typically 5.5 in intensively to high levels of aluminum, iron, and manganese weathered soils. in solution and to other factors associated with soil pH. This gives rise to the pH preferences of plants. Effect of pH on Soil Organisms The greater capacity of fungi, compared with bacteria, to thrive in highly acid soils was cited in pH Preferences of Plants Chapter 8. The pH requirement of some disease Blueberries are well known for their acid soil re- organisms is used as a management practice to quirement. In Table 11.6, the optimum pH range control disease. One of the best known examples for blueberries is 4 to 5. Other plants that prefer a is that of the maintenance of acid soil to control pH of 5 or less include: azaleas, orchids, sphag- potato scab. Potato varieties have now been denum moss, jack pine, black spruce, and cran- veloped that resist scab organisms in neutral and berry. In Table 11.6 the names of plants that prefer alkaline soils. Damping-off disease in nurseries is a pH of 6 or less are italicized. Most field and controlled by maintaining soil pH at 5.5 or less. garden crops prefer a pH of about 6 or higher. Earthworms are inhibited by high soil acidity. Because plants have specific soil pH require- Peter Farb (1959) relates an interesting case in ments, there is a need to alter or manage soil pH which soil pH influenced both earthworm and for their successful growth. mole populations. On a certain tennis court in England, each spring after rain and snow had washed away the court markings, the marking lines were still visible because moles had uner- MANAGEMENT OF SOIL pH ringly followed them. This intriguing mystery was investigated and produced the following solution. There are two approaches to assure that plants The tennis court had been built on acid soil and will grow without serious inhibition from unfa- chalk (lime) was used to mark the lines. As a vorable soil pH: (1) plants can be selected that result, the pH in the soil below the lines was grow well at the existing soil pH, or (2) the pH of increased. Earthworms then inhabited the soil un- the soil can be altered to suit the needs of the der the markings, and the soil remaining on the plants. Most soil pH changes are directed toward tennis court remained acid and uninhabitated by reduced soil acidity and increased pH by liming.

MANAGEMENT OF SOIL pH 179

The Lime Requirement acidity and to furnish calcium and magnesium for Agricultural lime is a soil amendment containing plant growth. Most frequently, agricultural lime is calcium carbonate, magnesium carbonate, and ground limestone that is mainly CaC0 3 and is other materials, which are used to neutralize soil calcitic lime. If the lime also contains a significant 180 SOIL CHEMISTRY

amount of MgC03 , the lime is dolomitic. The num toxicity in the subsoil that restricts rooting amount of lime needed or the lime requirement is depth and water uptake. Soils are droughty, which the amount of liming material required to produce is a difficult problem to solve, because the lime a specific pH or to reduce exchangeable and soil has low solubility and moves downward slowly. solution aluminum. Overliming the plow layer, to promote the down- ard movement of lime, may increase pH enough to cause micronutrient deficiences or toxicities. Lime Requirement of Intensively Weathered Soils Intensively weathered acid soils are characterized by low CEC that is highly aluminum Lime Requirement of Minimally and Moder- saturated and with low calcium and magnesium ately Weathered Soils Moderately and mini- saturation. The main benefits from liming are: mally weathered acid soils typically have a large (1) reduced exchangeable aluminum and re- CEC that has low aluminum saturation. Aluminum duced aluminum in solution, (2) an increase in toxicity and calcium deficiency are rare and, only the amount of XCa and/or XMg, and (3) increased occasionally, is there a magnesium deficiency. availability of molybdenum. The lime requirement is the amount needed to The lime requirement of intensively weathered increase or adjust pH to a more favorable value, as soils is the amount of liming material needed to shown in Figure 11.12 for production of soybeans. change the soil to a specific soluble aluminum Consider a moderately weathered soil with a content. This is commonly considered to be equal pH of 5.0 for the growth of soybeans, which have a to 1.5 times the amount of exchangeable alumi- pH preference of 6 to 7. The lime requirement is num. Thus, the lime requirement of the Rains soil the amount of lime required to adjust the pH to a (see Table 11.5) is equal to more favorable value for the crop being grown; for soybeans it would be about 6.5. The same is true for many other legumes, including alfalfa and The lime requirement for the Rains example is clover. Farmers who continuously grow corn can the amount of lime equal to 2.43 cmol of + charge maintain a lower soil pH than those producing per kilogram of soil. A mole of CaC03 weighs 100 soybeans and alfalfa, because corn is less respon- grams. Because of the divalent nature of the calcium, a half mole of CaC03, 50 g, will neutralize a mole of + charge. The amount of CaC03 equal to I cmol of + charge is 0.5 g (50 g/100). Therefore, FIGURE 11.12 Growth of soybeans versus soil pH. A pH of 6.5 is good for soybean production. 1.21 g/kg (2.43 cmol/kg x 0.5 g/kg) of CaC0 3 is the lime requirement. Assuming a 2 million kg furrow slice of soil per hectare, the lime needed is 2,420 kg/ha. The milliequivalent weight of CaC0 3 is 0.05 g. If CaC0 3 is used, the lime requirement is 0.12 g (2.43 meq x 0.05 g per meq) of CaC0 3 per 100 g of soil. Assuming an acre furrow slice weighs 2 million pounds, 2,400 pounds of CaC0 3 are required per acre. The rate must be adjusted to account for different thicknesses or weights of plow layers. In many soils, roots are susceptible to alumi- MANAGEMENT OF SOIL pH 181 sive to pH adjustment than are soybeans and al- tially CEC, as shown in Table 11.7. The data in falfa. Table 11.7 show lower lime requirements in the The amount of lime required to adjust the pH of southern coastal states, because the soils contain soils depends on the amount of pH adjustment clays (such as kaolinite) with lower CEC. needed and on the amount of buffer acidity. The amount of buffer acidity is related to the CEC. As shown in Figure 11.13, much less base (or lime), is required to increase the pH of Plainfield sand The Liming Equation and Soil Buffering than of Granby loam. In acid soils, H + in the soil solution comes mainly It is a common practice in soil testing laborato- from aluminum hydrolysis. Generally, a minor ries to make a direct measurement of the lime amount comes from XH unless there is a high requirement by using a buffer solution. A buffer organic matter content. Below pH 5.5, the hydro- solution with a pH of 7.5, for example, is mixed lysis is of exchangeable aluminum and, between with a known quantity of soil. After a standard 5.5 and 7, mainly from hydroxy-aluminum hydro- period of stirring, the pH of the buffer-soil suspen- lysis. The dominant aluminum form at pH 7 is sion is made. The amount of depression in pH AI(OH) 3 , which has very low solubility and, for all from that of the buffer solution is directly related practical purposes, is inert. Lime hydrolyzes in to the amount of lime needed to adjust soil pH to a the soil to form OH - : particular value. A less accurate but useful method, especially for the gardener or homeowner, is to determine The hydroxyls neutralize the H + that are produced soil pH with inexpensive color indicator dyes. The from aluminum hydrolysis, gradually the ex- lime requirement is obtained from a chart that changeable aluminum and hydroxy-aluminum relates lime need to soil pH and texture, essen- are converted to AI(OH) 3 , and the pH increases.

FIGURE 11.13 Titration curves showing the different amounts of base needed to increase the pH of soils having increasing cation exchange capacities from left to right. 182 SOIL CHEMISTRY

TABLE 11.7 Suggested Applications of Finely Ground Limestone to Raise the pH of a 7-inch Layer of Several Textural Classes of Acidic Soils, in Pounds per 1,000 ft2

The overall reaction representing the neutraliza- Acid soils that have been limed are still sub- tion of Al-derived soil acidity is jected to the natural acidifying effects of biological respiration, organic matter mineralization, and precipitation. Thus, soils again become acid and must be relimed about every 2 to 5 years, Exchangeable and hydroxy aluminum are precipi- depending on variables such as lime application tated as insoluble aluminum hydroxide, and the rate, CEC, rainfall, crop removal of calcium and amount of XCa is increased. The reduction in magnesium, and fertilization practices. acidic cations and increase in basic cations results in an increase in soil pH. Liming can be viewed as reversing the processes that produced Management of Calcareous Soils the soil acidity. Millions of acres of soil are calcareous in arid regions. In humid regions, many calcareous soils occur on floodplains and on recently drained lake Some Considerations in Lime Use plains. The pH ranges from about 7.5 to 8.3, and In agriculture, liming is usually done by applying many of these calcareous soils are excellent for ground limestone with spreaders, as shown in Figure 11.14. Major considerations are the purity, FIGURE 11.14 Broadcast application of lime on or neutralizing value, and fineness of the lime. acid soil. Calcium carbonate is quite insoluble, and particles that will not pass a 20-mesh screen are quite ineffective. It takes at least 6 months for lime to appreciably increase soil pH, and lime may be applied whenever it is convenient to do so. Cal- cium hydroxide (Ca(OH) 2 ) is also used as a liming material. It has greater neutralizing value per pound and is more soluble than the carbonate form. Wood ashes can also be used to increase soil pH. EFFECTS OF FLOODING ON CHEMICAL PROPERTIES 183 agricultural use. Plants growing on calcareous Oxygen serves as the dominant electron acceptor soils are sometimes deficient in iron, manganese, in the transformation of organic substrates to car- zinc, and/or boron. To lower soil pH would re- bon dioxide and water. When soils are flooded quire the leaching of the carbonates, which is naturally, or artificially as in rice production, a impractical. As a result, crops are fertilized with unique oxidation-reduction profile is created. A appropriate nutrients to overcome the deficien- thin, oxidized surface soil layer results from the cies, or crops that grow well on calcareous soil diffusion of oxygen from the water into the soil are selected. surface. This layer is underlain by a much thicker layer that is reduced (see Figure 4.11). Oxygen is Soil Acidulation deficient in the reduced zone, which initiates a The addition of acid sphagnum peat to soil may series of reactions that drastically alter the soil's have some acidifying effect. However, significant chemical environment. and dependable increases in soil acidity are probably best achieved through the use of sulfur. Sul- Dominant Oxidation and fur is slowly converted to sulfuric acid by soil Reduction Reactions microbes, and the soil slowly becomes more acid Flooded soils with a good supply of readily de- over a period of several months or a year. As with composable organic matter may become de- lime, the amount of sulfur required varies with the pleted of oxygen within a day. Then, anaerobic pH change desired and the soil CEC. Sulfur is and facultative microbes multiply and continue used to change soil pH in large agricultural fields the decomposition process. In the absence of ox- in arid regions, but it is less common a practice ygen, other electron acceptors begin to function, than the use of lime in humid regions. Sulfur is depending on their tendency to accept electrons. commonly used in nurseries and gardens. Rec- Nitrate is reduced first, followed by manganic ommendations for small areas are given in Table compounds, ferric compounds, sulfate and, fi- 11.8. nally, sulfite. The reactions are as follows (reduction to the right and oxidation to the left): EFFECTS OF FLOODING ON CHEMICAL PROPERTIES In well-aerated soils, the soil atmosphere contains an abundant supply of oxygen for microbial respiration and organic matter decomposition.

TABLE 11.8 Suggested Applications of Ordinary Powdered Sulfur to Reduce the pH of 100 ft 2 of a 20 Centimeter Layer of Sand or Loam Soil

From Kellogg, 1957. 184 SOIL CHEMISTRY

Weeks submerged

FIGURE 11.15 Flooding soil increases pH of acid soils and decreases the pH of alkaline soils. The pH changes are related to the soil's content of iron and organic matter. (Data from F. N. Ponnamperuma, 1976.)

Soils have various levels of reducing condi- dant electron acceptor (equation 3), and iron re- tions, depending on the quantity of decomposa- duction tends to increase soil pH. The production ble organic matter, the amount of oxygen brought of carbon dioxide, and the accompanying forma- in by water, and the temperature. The formation of tion of carbonic acid, have an acidifying effect. S 2- (sulfide) occurs in a strongly reduced envi- Alkaline soils tend to be low in available iron, ronment, resulting in the formation of FeS 2 and minimally weathered, and the production of car- H 2 S. In some instances, toxic amounts of H 2S are bonic acid lowers soil pH. In acid soils the abun- produced for rice production. About two weeks dance of soluble or available iron results in a net are needed after flooding to create rather stable increase in pH due to hydroxyl production. As a conditions. consequence, flooding of most rice fields causes the pH to move toward neutrality, as shown in Figure 11.15. There is an increase in nutrient avail- Changes in Soil pH ability, and soils generally do not need lime if they From the reactions just cited, there is a con- are flooded and puddled for rice production. The sumption of H + and an increase in hydroxyls. In increase in pH of highly acid soils is sufficient to most flooded rice fields, iron is the most abun- reduce the threat of aluminum toxicity. REFERENCES 185

SUMMARY Base Saturation Should Be Discouraged." Si Sci. Sc. A. J. 51:1089-1090. Madison, Wis. The kinds and amount of clay minerals and or- Clarke, F. W. 1924. "Data of Geochemistry." U.S. Ge. ganic matter determine the cation exchange ca- S. B. 770. Washington, D.C. pacity. Evans, C. E. and E. J. Kamprath. 1970. "Lime Response The abundance of adsorbed cations in neutral as Related to Percent Aluminum Saturation, Solution and slightly acid soils is Ca > Mg > K > Na. Aluminum, and Organic Matter Content." Soil Sci. These cations are held against leaching and are Soc. Am. Proc. 34:893-896. available for plant uptake. Farb, P. 1959. Liig Eah. Harper Row, New York. The anion exchange capacity is typically much Foth, H. D. 1987. "Proposed Designation Change for less than the cation exchange capacity, and it Exchangeable Bases." Ag Abac, Madi- son, Wis., pp. 1. results from protonation of hydroxyls. Foth, H. D. and B. G. Ellis. 1988. Si Feii. Wiley, Calcareous soils are alkaline as a result of hy- New York. drolysis of calcium carbonate. Leaching of the Foy, C. D. and G. B. Burns. 1964. "Toxic Factors in Acid carbonates in calcareous soils produces a pH of Si." Pa Fd Reie. V. 10. about 7. Continued leaching results in decreased Green, R. E., R. L., Fox, and D.D.F. Williams. 1965. "Soil amounts of exchangeable calcium, magnesium, Properties Determine Water Availability to Crops." potassium, and sodium. There is an increase in Haaii Fa Sciece. 14, (3):6-9. Hawaii Agr. Exp. exchangeable H and Al with increasing acidity. Sta., Honolulu. In acid mineral soils, H + for the soil solution Holzhey, C. S., R. B. Daniels, and E. E. Gamble. 1975. comes mainly from hydrolysis of aluminum, both "Thick Bh Horizons in the North Carolina Coastal Plain: 11 Physical and Chemical Properties and Rates hydroxy and exchangeable forms. Exchangeable of Organic Additions from Surface Sources." Si Sci. H+ increases as organic matter content increases Sc. A. Pc. 39:1182-1187. in acid soils. Krug, E. C. and C. R. Frink. 1983. "Acid Rain on Acid The lime requirement of intensively acid soils is Si." Sciece. 221:520-525. equal to about 1.5 times the amount of exchange- Jenny, Hans. 1941. Fac f Si Fai. McGraw- able aluminum. In minimally and moderately Hill, New York. weathered soils, the lime requirement is the Kellogg, C. E. 1957. "Home Gardens and Lawns." Si. amount of lime needed to increase soil pH to a USDA Yearbook. Washington, D.C. desired value. Kamprath, E. J. and C. D. Foy. 1985. "Lime and Fertilizer Sulfur is used to increase soil acidity. Interactions in Acid Soils. Feiie Techg ad Flooding soils results in a decrease in the pH of Ue. Am. Soc. Agron. Madison, Wis. alkaline soils and an increase in the pH of acid Ponnamperuma, F. N. 1976. "Specific Soil Chemical Characteristics for Rice Production in Asia." IRRI soils. Re. Pae Seie, No. 2. Manila. Spurway. C. H. 1941. "Soil Reaction Preferences of REFERENCES Plants." Mich. Ag. E. Sa. Sec. B. 306. East Lansing. Bohn, H. L., B. L. McNeal, and G. A. OConnor. 1985. Thomas, G. W. and W. L. Hargrove. 1984. "The Chem- Si Chei, 2nd ed. Wiley, New York. istry of Soil Acidity." In Si Acidi ad Liig. Binkley, D. 1987. "Use of the Terms Base Cation and Agronomy 12:1-58. Am. Soc. Agron. Madison, Wis. CHAPTER 12

PLANT-SOIL MACRONUTRIENT RELATIONS

For many centuries, people knew that manure, are translocated from leaves, stalks, and cobs and ashes, blood, and other substances had a stimu- are used for the development of the grain (see lating effect on plant growth. The major effect was Figure 12.1 for nitrogen). When the nutrient con- found to result from the essential elements content of leaves decreases below critical values dur- tained in these materials. Six of the essential nutri- ing the growing season, owing to translocation to ents are used in relatively large amounts and are other plant parts, specific symptoms develop. the macronutrients-usually over 500 parts per There is a considerable difference in the mobil- million (ppm) in the plant. Macronutrients in- ity of various nutrients within plants. A shortage of clude nitrogen, phosphorus, potassium, calcium, a mobile nutrient, such as nitrogen, results in the magnesium, and sulfur. A list of the macronu- translocation of the nutrient out of the older and trients and their major roles in plant growth first formed tissue into the younger tissues. This are given in Table 12.1. The quantities of the causes the deficiency symptoms to appear first on macronutrients contained in harvested crops are the older or lower leaves. This is true for nitrogen, given in Table 12.2. phosphorus, potassium, and magnesium. Immo- bile plant nutrients cannot be remobilized when a shortage of the nutrient occurs, resulting in a deficiency in the most recent or newly formed DEFICIENCY SYMPTOMS tissues. Macronutrients with limited mobility in Translocation of nutrients within a plant is an plants are calcium and sulfur. ongoing process. Early in the gowing season, a corn plant consists mainly of leaves, and the nitrogen exists mostly in the leaves, as shown in Figure NITROGEN 12.1. As the growing season continues, considerable nitrogen accumulates in the stalks and cobs. The atmosphere is made up of 79 percent nitro- The grain (fruit) develops last and has a high gen, by volume, as inert N2 gas. Although a large priority for the nutrients within the plant. In the quantity of nitrogen exists in the atmosphere, the latter part of the growing season, mobile nutrients nutrient that is absorbed from the soil in the great-

186

NITROGEN 187

TABLE 12.1 Essential Macronutrient Elements and The N2 is characterized by both an extremely Role in Plants strong triple bonding between the two nitrogen atoms and a great resistance to react with other Element Role in Plants elements. The process of converting N 2 into forms Nitrogen (N) Constituent of all proteins, that vascular plants can use is termed nitrogen chlorophyll, and in coenzymes, fixation, (step 1 of Figure 12.2). The other major and nucleic acids. steps in the soil nitrogen cycle are: (2) minerali- Phosphorus (P) Important in energy transfer as part of adenosine triphosphate. zation, (3) nitrification, (4) immobilization, and Constituent of many proteins, (5) denitrification. coenzymes, nucleic acids, and metabolic substrates. Potassium (K) Little if any role as constituent of plant compounds. Functions in Dinitrogen Fixation regulatory mechanisms as Dinitrogen fixation is the conversion of molecular photosynthesis, carbohydrate nitrogen (N2 ) to ammonia and subsequently into translocation, protein organic forms utilizable in biological processes. synthesis, etc. Some N2 is fixed by lightning and other ionizing Calcium (Ca) Cell wall component. Plays role phenomena of the upper atmosphere. This fixed in the structure and permeability of membranes. nitrogen is added to soils in precipitation. Magnesium (Mg) Constituent of chlorophyll and Most nitrogen fixation is biological, being ei- enzyme activator. ther symbiotic or nonsymbiotic. Nitrogen-fixing Sulfur (S) Important constituent of plant organisms contain an enzyme, nitrogenase, proteins. which combines with a dinitrogen molecule (N 2 ) Complied from many sources. and fixation occurs in a series of steps that reduces N 2 to NH3 , as shown in Figure 12.3. Molyb- denum is a part of nitrogenase and is essential for est quantity and is the most limiting nutrient for biological nitrogen fixation. The nitrogen-fixing food production is nitrogen. organisms also require cobalt, which is an example of plants need for cobalt.

The Soil Nitrogen Cycle The nitrogen cycle consists of a sequence of bio- Symbiotic Legume Fixation Legume plants chemical changes wherein nitrogen is used by are dicots that form a symbiotic nitrogen fixation living organisms, transformed upon death and de- relationship with heterotrophic bacteria of the ge- composition of the organisms, and converted ulti- nus Rhizobiurn. When the root of the host plant mately to its original state of oxidation. The major invades the site of a dormant bacterial cell, a segments of the soil nitrogen cycle are shown as so-called "recognition event" occurs. A root hair five steps or processes in Figure 12.2. in contact with the bacterial cell curls around the The reservoir of nitrogen for plant use is essen- cell. The dormant bacterial cell then germinates tially that in the atmosphere, N2 . There is virtually and forms an infection thread that penetrates the an inexhaustible supply of nitrogen in the atmo- root and migrates to the center of the root (see sphere, since (at sea level) there are about 77,350 Figure 12.4). Once inside the root, bacteria multi- metric tons of N 2 in the air over I hectare (34,500, ply and are transformed into swollen, irregular- tons per acre). It takes about a million years for shaped bodies called bacteroids. An enlargement the nitrogen in the atmosphere to move through of the root occurs and, eventually, a gall or nodule one cycle. is formed (see Figure 8.12). The bacteroids re- 188 PLANT-SOIL MACRONUTRIENT RELATIONS

TABLE 12.2 Approximate Pounds per Acre of Macronutrients Contained in Crops

Phosphorus Potassium Acre Crop Yield Nitrogen as P as K Calcium Magnesium Sulfur Grains Barley (grain) 40 bu. 35 7 8 1 2 3 Barley (straw) I ton 15 3 25 8 2 4 Corn (grain) 150 bu. 135 23 33 16 20 14 Corn (stover) 4.5 tons 100 16 120 28 17 10 Oats (grain) 80 bu. 50 9 13 2 3 5 Oats (straw) 2 tons 25 7 66 8 8 9 Rice (grain) 80 bu. 50 9 8 3 4 3 Rice (straw) 2.5 tons 30 5 58 9 5 Rye (grain) 30 bu. 35 5 8 2 3 7 Rye (straw) 1.5 tons 15 4 21 8 2 3 Sorghum (grain) 60 bu. 50 11 13 4 5 5 Sorghum (stover) 3 tons 65 9 79 29 18 Wheat (grain) 40 bu. 50 11 13 1 6 3 Wheat (straw) 1.5 tons 20 3 29 6 3 5 Hay Alfalfa 4 tons 180 18 150 112 21 19 Bluegrass 2 tons 60 9 50 16 7 5 Coastal Bermuda 8 tons 185 31 224 59 24 - Cowpea 2 tons 120 11 66 55 15 13 Peanut 2.25 tons 105 11 79 45 17 16 Red clover 2.5 tons 100 11 83 69 17 7 Soybean 2 tons 90 9 42 40 18 10 Timothy 2.5 tons 60 11 79 18 6 5 Fruits and vegetables Apples 500 bu. 30 5 37 8 5 10 Beans, dry 30 bu. 75 11 21 2 2 5 Cabbage 20 tons 130 16 108 20 8 44 Onions 7.5 tons 45 9 33 11 2 18 Oranges (70 pound boxes) 800 boxes 85 13 116 33 12 9 Peaches 600 bu. 35 9 54 4 8 2 Potatoes (tubers) 400 bu. 80 13 125 3 6 6 Spinach 5 tons 50 7 25 12 5 4 Sweet potatoes (roots) 300 bu. 45 7 62 4 9 6 Tomatoes (fruit) 20 tons 120 18 133 7 11 14 Turnips (roots) 10 tons 45 9 75 12 6 Other crops Cotton (seed and lint) 1,500 lb 40 9 13 2 4 2 Cotton (stalks, leaves, and burs) 2,000 lb 35 5 29 28 8 NITROGEN 189

TABLE 12.2 (continued) Phosphorus Potassium Acre Crop Yield Nitrogen as P as K Calcium Magnesium Sulfur Peanuts (nuts) 1.25 tons 90 5 13 1 3 6 Soybeans (grain) 40 bu. 150 16 46 7 7 4 Sugar beets (roots) 15 tons 60 9 42 33 24 10 Sugarcane 30 tons 96 24 224 28 24 24 Tobacco (leaves) 2,000 lb 75 7 100 75 18 14 Tobacco (stalks) 35 7 42

Data from Plant Food Review, 1962, pp. 22-25, publication of The Fertilizer Institute. a These values are about the same as kilograms per hectare (multiply pounds per acre by 1.121).

ceive food, nutrients, and probably certain growth plants. Weber (1966) used this method and found compounds from the host plant. The fixed nitro- that as much as about 160 kilograms of nitrogen gen in the nodule is transported from the nodule per hectare (140 lb per acre) was fixed when to other parts of the host plant, which is beneficial soybeans were grown. This is shown in Figure for the host. 12.5 for the zero amount of nitrogen applied per One method used to study the amount of nitro- hectare. In addition, about 75 percent of the nitrogen that is fixed is to compare the amount of gen in the tops of the soybean plants was derived nitrogen in nodulated and nonnodulated legume from nitrogen that was fixed, and 25 percent was

FIGURE 12.1 Growth and nitrogen in corn plants. Part of the nitrogen taken up and present in the leaves, stalks, and cobs early in the growing season is later moved to other organs. The grain has high priority for the nitrogen as shown by the large amount in the grain at harvest time. (From J. Hanway, 1960.) 190 PLANT-SOIL MACRONUTRIENT RELATIONS

FIGURE 12.2 The major processes of the soil nitrogen cycle are fixation, mineralization, nitrification, immobilization, and denitrification. Nitrogen can also be lost from the soil by leaching and volatilization.

FIGURE 12.3 A simplified series of steps in nitrogen, fixation. (Adapted from Delwiche, in The Science Teacher, Vol. 36, p. 19, March 1969.) NITROGEN 19 1

FIGURE 12.4 Early stages in nodule formation. (a) Recognition event between the root hair and bacteria. (b) Curling of root hair. (c) Early penetration of the infection thread. (From P. W. Wilson, The Biochemistry of Symbiotic Nitrogen Fixation, The University of Wisconsin Press, 1940, p. 74. Used by permission of P. W. Wilson and the University of Wisconsin Press.)

derived from the soil. Also note in Figure 12.5 that the soil. From these and other data, it is reason- the amount of fixed nitrogen decreased as the able to conclude that 50 to 200 kilograms per amount of nitrogen applied to the soil as fertilizer hectare (or pounds per acre) are commonly fixed was increased. At the highest nitrogen fertilizer annually when legumes such as soybeans, clover, rate, little nitrogen fixation occurred, and nearly or alfalfa are grown. Peas and navy beans fix less all of the nitrogen in the plant was absorbed from nitrogen, so it is a standard practice to use nitrogen fertilizers to increase pea and bean yields. Many trees are legumes and fix nitrogen. Black FIGURE 12.5 Amount of total nitrogen fixed and locust is an example of a nitrogen-fixing legume percent of nitrogen in soybean plants from symbiotic fixation in relation to the application of nitrogen tree. In the tropics, Leucaena trees have been fertilizer. (Data from C. R. Weber 1966.) used to supply nitrogen for food crops in mixed cropping systems. The trees are also effective for erosion control. Other additional benefits of the Leucaena are that the leaves can be used as forage for animals and the wood can be used for fuel. The same benefits are obtained from the growth of cowpeas, which forms a small shrub. In actual farming practice, the amount of nitrogen added to soils by the growth of legume crops depends on the amount of the crop that is harvested and removed from the soil. If an entire crop of legumes is plowed under, all the fixed nitrogen is added to the soil. If all of the legume crop is harvested and removed from the field, there may 192 PLANT-SOIL MACRONUTRIENT RELATIONS actually be a net loss of soil nitrogen by the pro- symbiotically, and a dozen or more have been duction of the legume crop. This occurs when found. The two organisms that have been studied soybeans are grown and only the beans are har- most belong to the genus Azotobacter and the vested. Legume residues-roots, stems and genus Clostridium. leaves-are relatively rich in nitrogen and are de- Azotobacter are widely distributed in nature. composable (mostly labile). Even though the pro- They have been found in soils with pH 6 or above duction of a legume crop may not increase total in almost every instance where examinations soil nitrogen, the decomposition of the plant resi- have been made. The greatest limiting factor af- dues releases considerable available nitrogen fecting their distribution in soil appears to be pH. during the following months. These organisms are favored by pH 6 or more, good aeration, abundant organic matter, and suitable moisture. Clostridium is also a heterotrophic Nonlegume Symbiotic Fixation Many nonle- bacteria, but is an anaerobe that can thrive in soils gume plant species have root nodules and also fix too acid for Azotobacter. It exists mostly as a nitrogen symbiotically. This means that symbiotic spore and becomes vegetative for brief periods fixation of nitrogen is important in natural ecosys- after rains when anaerobic conditions exist. Both tems by both legume and nonleguminous plants. of these heterotrophs depend on decomposable Red alder is an example of a nonlegume capable organic matter for their energy supply and thus, of symbiotic nitrogen fixation. This feature makes have limited activity in soils and contribute little red alder (Alnus) a good pioneer species for in- nitrogen for plant growth in cultivated fields. vading freshly exposed parent materials and lands that are burned over, where soils have low ni- Summary Statement In every ecological niche trogen-supplying capacity because of low organic there are nitrogen fixing organisms and systems matter content. Ceanothus species (a flowering that transfer N2 from the atmosphere to the soil. shrub), in the forests of the U.S. Pacific Northwest, Plants that fix nitrogen are good pioneers for the fixes a large amount of nitrogen. The nitrogen- colonization of parent materials and soils that are fixing organisms in this nonlegume are actino- low in organic matter and nitrogen. In cultivated mycetes. The nonlegume symbiotic nitrogen- fields, legume-Rhizobia symbiotic fixation of ni- fixing systems result in more fixation of nitrogen trogen has traditionally been a primary source of in natural ecoystems than in the symbiotic herba- nitrogen in agriculture. The amount of nitrogen ceous legumes found in agricultural fields. fixed symbiotically in most cultivated soils is re- In water, the fern, Azolla, forms an association lated to the legume species, the nitrogen-fixing with blue-green algae. The blue-green algae live Rhizobia species, and the amount of nitrogen in the fronds of the fern and fix nitrogen. The algal available in the soil. Whether or not the produc- partner can fix all the nitrogen that the association tion of a legume crop will increase the total con- can use. Prolific growth of Azolla forms a floating tent of soil nitrogen is dependent on the amount mat of biomass on the surface of the water in rice of the crop that is harvested and removed from the paddies and other bodies of water. In Asia, Azolla field. Azolla fixes nitrogen symbiotically and is is cultivated, and the biomass is harvested and cultivated in Asia. The biomass is harvested and is used as a manure for crop production and as used as manure in crop production and is used as animal feed. animal feed.

Nonsymbiotic Nitrogen Fixation Certain Mineralization groups of bacteria living independently in the soil At any given time, about 99 percent of soil nitro- can fix nitrogen. These bacteria fix nitrogen non- gen is organic nitrogen in organic matter. About 1 NITROGEN 193 to 2 percent of the total organic nitrogen is de- plants depends on the amount of nitrogen miner- composed and the nitrogen mineralized each alized, which is dependent on the kinds and year. Many different kinds of heterotrophic organ- amounts of soil organic matter and the rate of isms are involved. The first inorganic or mineral nitrogen mineralization. form of nitrogen produced by decomposition The total nitrogen in a grassland soil in Canada (mineralization) is ammonia (NH 3 ). For this rea- was divided into four pools, depending on de- son, the nitrogen mineralization process has also composability, as shown in Table 12.3. The living been called ammonification; (see step 2 in Figure biomass accounted for only 4 to 6 percent of the 12.2). soil nitrogen, but contributed 30 percent to the Some NH 3 volatilizes from animal droppings or amount of nitrogen mineralized in a 12-week pe- manure, and some NH 3 produced at or near the riod. By contrast, the oldest and most stable nitro- soil surface escapes into the atmosphere by vola- gen compounds accounted for 50 percent of the tilization, especially when soil pH is 8 or more. total soil nitrogen and contributed a mere 1 per- Ammonia is a polar gas and, in the soil, NH 3 recent to the mineralized nitrogen (see Table 12.3). acts with water and then combines with a H+ to Each percent of soil organic matter represents form ammonium, NH4'. Ammonmium is ad- about 1,000 pounds of nitrogen per acre-furrow- sorbed onto the cation exchange sites and is slice, assuming soil organic matter is 5 percent retained in soils as an exchangeable cation. nitrogen. If 1 percent of the total organic nitrogen The ammonium ion is about the same size as is mineralized each year, the nitrogen mineraliz- the potassium ion, and some NH4' becomes en- ing potential of the plow layer is 10 pounds per trapped in the interlayer space of micaeous min- acre annually for each percent of organic matter. erals, somewhat like the entrappment of K + . This A plow layer containing 3 percent organic matter entrappment of ammonium nitrogen is called am- would annually mineralize about 30 pounds of monium fixation. Fixed NH 4 ' has low plant avail- nitrogen per acre. ability. It appears that all plants obtain some of their nitrogen from the soil. Plants that participate in Nitrification symbiotic nitrogen fixation may obtain the great Nitrification is the biological oxidation of ammo- bulk of their nitrogen through fixation. Plants that nium (NH4') to nitrite (N02 ) and to nitrate - do not have a symbiotic nitrogen-fixing relation- (N03 ). This process is shown as step 3 in Figure ship must meet all of their nitrogen needs from the 12.2. Nitrification is a two-step process, with ni- - soil. How much nitrogen the soil can supply trite (NO 2 ) as the intermediate product. Specific

TABLE 12.3 Nitrogen Pools and Their Contribution to Mineralized Nitrogen in a Chernozemic (grassland) Soil

° Time for one-half of the material to decompose. 194 PLANT-SOIL MACRONUTRIENT RELATIONS autotrophic bacteria are involved. The reactions per year is usually necessary for lawns and other and organisms are as follows. long-season crops to maintain vigorous growth.

Nitrate is subject to leaching, whereas, NH4' is adsorbed onto the cation exchange sites and resists leaching. This has important implications for the economy of use of nitrogen fertilizer and nitrate pollution of groundwater. During the winter The nitrifying bacteria obtain energy from these and spring, plants may be dormant and not be oxidizing reactions. Note also that the first step in active in nitrogen uptake. This is the time of year nitrification produces H+, which contributes to when surplus water and groundwater recharge are soil acidity. Nitrification changes the form of likely to occur and nitrate leaching is most likely. available nitrogen from a cation to an anion. This Nitrogen fertilizer applied in the fall is usually as is of no great concern in plant nutrition, since ammonium, commonly with a nitrification inhibi- most plants readily use both nitrate and am- tor. The nitrification inhibitor is used to maintain monium. the nitrogen in the ammonium form and reduce Nitrification is affected by oxygen supply and possible leaching loss of nitrogen as nitrate, and soil pH. Nitrification is inhibited in water- this reduces the danger of groundwater pollution. saturated soils. Plants that are native to water- saturated environments, where the lack of 0 2 does not permit nitrification to occur, tend to Immobilization prefer ammonium. Paddy rice is more productive Both ammonium and nitrate are available forms when an ammonium fertilizer rather than a nitrate of nitrogen for roots and microorganisms. Their fertilizer is applied. uptake and use result in the conversion of inor- When soils have a pH of 6 or more and are well ganic (mineral) forms of nitrogen into organic aerated, there is an abundance of nitrifying form. The process is immobilization (shown as bacteria-bacteria that quickly oxidize NH4' to step 4 in Figure 12.2). Immobilized nitrogen is N03 - . Well-aerated soils with pH of 6 or more stable in the soil, in the sense that is is not readily also provide an environment that is favorable for lost from the soil by leaching, volitalization, or production of most economic crops. Thus, condi- denitrification, and is not taken up by roots and tions favorable for crop production are favorable microorganisms. Soil nitrogen is subject to re- for nitrification, resulting in N03 being a com- peated cycling within the soil through mineraliza- mon form of nitrogen absorbed by plants and tion, nitrification, and immobilization. microorganisms. Mineralization and immobilization are two op- In most well-aerated soils the dominant form of posing processes that generally control the level available nitrogen is nitrate. It remains soluble in of available nitrogen that exists in the soil. As the soil solution and moves readily to roots in noted earlier, a soil with 3 percent organic matter water by mass flow. In essence, plants can rapidly might mineralize 30 pounds of nitrogen per acre absorb both the water and nitrate from a soil if per year. Part of this nitrogen is used by the miner- both are in good supply (available). This has two alizing organisms. Perhaps, only one half of the important consequences. First, mass flow is the amount of nitrogen mineralized might be in ex- major means by which nitrogen is moved to root cess of that needed by the microorganisms. This surfaces for absorption. It accounts for about 80 would result in only 15 pounds of nitrogen avail- percent of the nitrogen absorbed by corn. Second; able for uptake by roots. Large crop yields may the nitrogen from fertilizer is quickly used, and contain several hundred pounds of nitrogen per more than one application of nitrogen fertilizer acre, which means that large amounts of nitrogen NITROGEN 195 fertilizer are commonly applied to produce non- ing plants, and means that plants may be starved leguminous crops. for nitrogen for a period of several weeks. Eventu- ally, the straw material will be consumed and the carbon and nitrogen assimilated end up as live Carbon-Nitrogen Relationships Some plant- and/or dead microbial tissue. This tissue, along residue materials such as wheat straw, corn with other SOM (with relatively small C : N ratios) stover, and sawdust have a very small nitrogen become the major substrate for the decomposers, content relative to their carbon content. As a con- and the temporary period of nitrogen starvation sequence, the ratio of carbon to nitrogen (C : N ceases. These changes are illustrated in Figure ratio) is relatively large compared with soil humus 12.6. and the leguminous plants, which tend to be rich In general, materials with C : N ratios between in nitrogen (see Table 12.4). Bacteria require 20 and 30 have just about enough nitrogen to about 5 grams of carbon for for each gram of satisfy the needs of the decomposers. Materials nitrogen assimilated or use carbon and nitrogen with higher ratios produce a temporary period of in a ratio of 5:1. Consequently, when straw is nitrogen deficiency for plant growth. Humus, added to the soil, there is insufficient nitrogen to animal manure, and legume residues are rela- meet the needs of the decomposing organisms. tively rich in nitrogen, have a relatively small C:N Under these conditions, decomposition of the ratio, and contain more nitrogen than the decom- straw is limited by an insufficient supply of nitro- posers need. These materials with small C:N ra- gen; unless, nitrogen is available from some other tios are excellent materials to add to soils to in- source. This other source could be nitrogen min- crease the available nitrogen for growing plants. eralized within the soil from SOM. The decom- An addition of about 20 pounds of nitrogen to posing microorganisms have first priority for any the soil as fertilizer per ton of straw is required to mineralized nitrogen (nitrogen released in de- prevent a temporary nitrogen deficiency, as a re- composition). When there is insufficient nitrogen sult of the microbial decomposition of the straw. in the straw to supply the needs of the decom- When a large amount of straw or other similar posers, they will use any nitrogen that is available. material is added to soils without supplemental This results in a deprivation of nitrogen for grow- nitrogen fertilizer, a month or so is required for the temporary nitrogen-deficient period to pass under conditions favorable for decomposition.

Denitrification Denitrification is the reduction of nitrate or nitrite to molecular nitrogen or nitrogen oxides by microbial activity. Many of the denitrification products are gaseous and escape from the soil. Just as it is natural for nitrogen to be added to soils by fixation, it is natural for nitrogen to be lost from the soil by denitrification. In ecosystems where the total amount of nitrogen remains constant from year to year, the annual additions of nitrogen via fixation tend to balance the losses of nitrogen Data are taken from several sources. The values are approxi- by denitrification. Thus, denitrification is one of mates only, and the ratio in any particular material may vary considerably from the values given. the most significant processes in the nitrogen cy- 196 PLANT-SOIL MACRONUTRIENT RELATIONS

FIGURE 12.6 Addition of organic material with high C : N ratio results in depletion of soil nitrate. The activity of the decomposers is greatly stimulated and they use whatever nitrogen is present. In time, most of the added material is decomposed and the soil nitrate level increases. cle, because denitrification accounts for large Near Atlanta, Georgia, there is a large sewage losses of nitrogen from soils. disposal project that uses sprinklers to dispose of Denitrification is carried out by certain faculta- the effluent in a forest. The rate of effluent applica- tive and anaerobic organisms. Nitrate-nitrogen in tion is controlled to produce an anaerobic soil poorly drained soil is especially subject to denitri- environment and denitrification of the nitrates in fication. Anaerobic conditions may exist in well- the effluent before the water leaves the disposal drained soils after rains, when the interiors of soil area. This use of denitrification is likely to in- aggregates are wet. Denitrification is shown as crease as more and more communities become step 5 in Figure 12.2, and the reaction is as involved in disposing of sewage effluent without follows. contaminating groundwater, and at the same time, contributing water to groundwater recharge. C6H 1 2 0 6 + 4NO3 = 6CO2 + 6H 2 0 + 2N 2 Nitrite (NO2 - ), nitric oxide (NO), and nitrous oxide (N 20) are also produced. Human Intrusion in the Nitrogen Cycle Normally, denitrification is detrimental to agri- The amount of nitrogen in a corn crop is large culture, because nitrogen is lost from soils. Deni- (see Table 12.2) compared to the nitrogen- trification, however, helps to prevent an excess of supplying power of soils. The United States pro- nitrate in the groundwater of irrigated valleys duces more than 50 percent of the world's corn, where high rates of nitrogen fertilizer have been and this requires a large amount of nitrogen fertil- used. In a large irrigated valley, denitrification izer. The balance sheet for the earth's nitrogen occurs as nitrate in shallow groundwater moves cycle in Figure 12.7 shows that the amount of slowly down the length of the valley. This denitri- industrially fixed nitrogen was equivalent to the fication contributes to a reduced nitrate content of terrestrial (natural) fixation and about twice as the drainage water that exits the valley, and thus great as the amount of nitrogen fixed by legume less threat of nitrate pollution. Where nitrate is crops. The biosphere now receives about 11 per- carried downward below the biologically active cent more nitrogen than is lost. It is obvious that zone where denitrifers and an energy source exist, humans have become an important factor in the denitrification does not occur. Thus, nitrate in nitrogen cycle through industrial nitrogen fixa- deep ground water is not denitrified. tion. The long-term effects of a nitrogen buildup in PHOSPHORUS 197

increase the nitrogen content of soils because of the natural, large losses of nitrogen by denitrification (and also leaching and volatilization). Within the soil there is an internal nitrogen cycle that shuttles nitrogen back and forth through mineralization, nitrification, and immobilization. The amount of nitrogen available for crops is the nitrogen in excess of the needs of the mineralizing microorganisms. The low nitrogen- supplying power of soils results in large additions of nitrogen to soils as fertilizers to meet the nitrogen needs of high-yielding nonleguminous crops. The large amount of industrially fixed nitrogen has resulted in a significant intrusion in the earth's nitrogen cycle by humans.

Plant Nitrogen Relations Nitrogen is a part of chlorophyll, and nitrogen- deficient plants have light-green or yellow leaves. Nitrogen-deficient plants are stunted, producing low crop yields. The yellowish color of lawns in the growing season is usually an indication of nitrogen deficiency. Many plants develop distinct nitrogen-deficiency symptoms, including yellow midribs of the lower leaves of corn (maize) plants, FIGURE 12.7 Nitrogen balance sheet for the world's as shown in Color Plate 1. Yellow and nitrogen- biosphere. Nine million metric tons (92-83) more deficient corn leaves have at low nitrate content, nitrogen is being added than is being removed by denitrification (and loss to sediments). (Data from according to tissue analysis (see Color plate 1). C. C. Delwiche, "The Nitrogen Cycle, " 1970, Scientific An excess of nitrogen causes rapid vegetative American, Inc. All rights reserved.) growth and dark-green leaves. The vegetation is succulent and has reduced resistance to injury the biosphere are unknown. There is, however, from insects, disease, and frost. For some crops, increased potential for groundwater pollution and succulent tissue means greater damage from me- eutrophication of lakes. In agriculture, as long as chanical harvesting and reduced storability. Ce- the use of nitrogen fertilizer does not result in real grains develop tall, weak stalks that are easily adding more nitrogen than crops and microor- blown over or broken during rainstorms, when the ganisms can immobilize, there is little danger of heads are heavy and filled with grain near harvest groundwater pollution. time.

PHOSPHORUS Summary Statement on Nitrogen Cycle For the earth there is a quasi-equilibrium between The earth's crust contains about 0.1 percent phos- fixation and denitrification that maintains a rather phorus (P). On this basis, the phosphorus in a constant soil nitrogen content. It is difficult to plow layer is equivalent to the phosphorus in 198 PLANT-SOIL MACRONUTRIENT RELATIONS

- 20,000 bushels of corn for an acre. This does not phate ion, H 2 P04 , which exists in the soil solu- include phosphorus that could be absorbed by tion, as shown as step I of Figure 12.8. The - roots at depths below the plow layer. Phosphorus, H 2 P0 4 is immobilized when roots and microor- however, commonly limits plant growth. The ma- ganisms absorb it and convert the phosphorus jor problem in phosphorus uptake from soils by into organic compounds. This results in a signifi- roots is the very low solubility of most phosphorus cant amount of phosphorus in soils as organic compounds, resulting in a low concentration of phosphorus. Commonly, 20 to 30 percent of the phosphate ions in the soil solution at any one phosphorus in plow layers of mineral soils is time. organic phosphorus. As with nitrogen, organic phosphorus is mineralized by microorganisms and is again released to the soil solution as - Soil Phosphorus Cycle H 2 P04 . The organic phosphorus cycle, a sub- Most phosphorus occurs in the mineral apatite in cycle in the overall soil phosphorus cycle, is simi- igneous rocks and soil parent materials. Fluora- lar to the soil nitrogen cycle, where phosphorus is patite (Ca 1o (P04 ) 6 F2) is the most common apatite shuttled back and forth by mineralization and im- mineral. Fluorapatite contains fluorine (F), which mobilization (steps 2 and 3 in Figure 12.8). contributes to a very stable crystalline structure A major difference between the nitrogen and that has great resistance to dissolution or weath- phosphorus cycles in soils is that the available ering. The structure is similar to that of teeth den- form of nitrogen is mainly nitrate and is stable in tine. Fluoridation of water and tooth paste is de- the soil solution. Nitrate remains in solution and signed to incorporate fluorine into dentine to moves rapidly to roots by mass flow, unless deni- increase resistance to tooth decay. trified or leached. The phosphate ion, by contrast, Apatite weathers slowly, producing the phos- quickly reacts with other ions in the soil solution,

FIGURE 12.8 Major processes in the soil phosphorus cycle. The availability of phosphorus to plants is determined by the amount of phosphorus in the soil solution. PHOSPHORUS 199 resulting in precipitation and adsorption to min- fixed by adsorption onto clays and other soil con- eral colloids that convert the phosphorus to an stituents. Fixed phosphorus dissolves or is re- unavailable or fixed form (step 4 in Figure 12.8). leased slowly into the soil solution (step 5 of As a consequence, most of the phosphate ions Figure 12.8). The very low concentration of phos- from mineralization of organic phosphorus, or phorus in the soil solution results in (1) very little mineral weathering, may be converted to an un- phosphorus being moved to roots by mass flow, available form before plants have an opportunity and (2) minimal loss of phosphorus from soils by to absorb the phosphorus and before loss by leaching. The optimum pH for phosphorus avail- leaching can occur. The kinds of ions in the soil ability is near 6.5, where there is the least poten- solution that render phosphate insoluble are re- tial for phosphorus fixation. Even at this pH only a lated to soil pH. very small amount of phosphorus is found in the soil solution, the amount in solution being of the order of 0.04 pounds per acre-furrow-slice. Effect of pH on Phosphorus Availability Newly formed aluminum and iron phosphates The ions in the soil solution are a function of pH. are amorphous (not crystalline) and have a large In calcareous parent materials and soils, the pH is surface area per gram. With time, the amorphous commonly in the 7.5 to 8.3 range. In these soils, forms become more crystalline, as shown in Fig- apatite is the dominant phosphorus mineral. ure 12.9. There is a large decrease in surface area Apatite has very low solubility, and even plants in with increasing crystallinity, and this is associated virgin soils being cropped for the first time, have with decreased solubility or availability of the aluminum and iron phosphate minerals. In an exper- difficulty satisfying their phosphorus 21needs. The ions in the soil solution include Ca from the iment with Sudan grass grown in quartz sand, hydrolysis of calcium carbonate. Any phosphate plants growing in soil that received crystalline 2- ions released, predominantly HP0 4 above pH forms of iron and aluminum phosphate grew 7.2, tend to precipitate as insoluble calcium phos- somewhat like plants that had not received phosphate compounds that are slowly converted to phate. By contrast, the plants growing in soil that apatite. Therefore, in calcareous soils most of the received colloidal iron or aluminum phosphate phosphorus tends to exist as apatite and tends grew much better and absorbed much more phos- to remain as apatite. Phosphorus availability to phorus, as shown in Table 12.5. plants is low. Over the course of soil evolution in humid regions, the calcium carbonate dissolves, the cal- Changes in Soil Phosphorus Over Time cium is leached, and the soils become acid. In The forms of phosphorus in soils change during t+ the long period of soil evolution. In young cal- acid soils, there 31is much less Ca and much more Al 3+ and Fe in solution than in calcareous careous soil, the phosphorus is mainly apatite soils. This results in precipitation of phosphate phosphorus. Plant growth and organic matter ac- ions as insoluble aluminum and iron phosphates. cumulation in the soil result in the conversion of a Again, these phosphorus compounds have very significant amount of apatite phosphorus into or- low solubility, resulting in a low concentration of ganic phosphorus. As soils become acid, phos- phosphate ions in solution. phorus is fixed as iron and aluminum phosphate. The formation of these insoluble calcium, alu- In acid soils, phosphorus released by dissolution minum, and iron compounds is called phos- of apatite is gradually converted into iron and phorus fixation (step 4 in Figure 12.8). In the case aluminum phosphates. When first formed, the of phosphorus, the fixation, is usually the result of iron and aluminum phosphates are amorphous precipitation or adsorption. Phosphate ions are and, over time, they gradually become crystalline. 200 PLANT-SOIL MACRONUTRIENT RELATIONS

FIGURE 12.9 Transformation of soluble phosphorus into less available forms. Availability decreases with increases in crystallinity. (From A. S. R. Juo, and B. G. Ellis, 1968. "Chemical and Physical Properties of Iron and Aluminum Phosphates and Their Relation to Phosphorus Availability." Soil Sci. Soc. Am. Proc. 32:216-221. Used by permission of the Soil Science Society of America.)

Eventually, in intensively weathered soils rich in there is no occluded phosphorus. By contrast, the iron and aluminum oxides, the crystalline forms intensively weathered soil from Hawaii, has no of iron and aluminum phosphate become encap- calcium phosphate, and a small amount of iron sulated (occluded) by iron and aluminum oxides. and aluminum phosphorus, but the phosphorus Occluded phosphorus is the least available form is mainly occluded. These soils have the least of soil phosphorus. Note in Figure 12.10 that most ability to supply plants with phosphorus. The of the phosphorus is calcium phosphate in a mini- moderately weathered Wisconsin soil is interme- mally weathered North Dakota soil, little phos- diate and is the best suited for supplying phosphorus is aluminum and iron phosphate, and phorus to plants.

Plant Uptake of Soil Phosphorus TABLE 12.5 Yield and Phosphorus Uptake by Sudan The dominant form of phosphorus available to grass after Growing 30 Days in Quartz Sand Culture - plants exists in the soil solution mainly as H 2 PO 4 2- below pH 7.2 and mainly as HPO 4 above pH 7.2. The potential for P fixation tends to maintain phosphorus in insoluble forms and, therefore, the phosphorus concentration in the soil solution is very low. Thus, mass flow cannot supply plants with sufficient phosphorus except in unusual - cases. As a result, most of the H 2 PO4 at root surfaces has been moved there by diffusion. It has Adapted from Juo and Ellis, 1968. been reported that about 90 percent of the phos- a Relative to colloidal aluminum phosphate as 100. phorus in corn is supplied by diffusion and about PHOSPHORUS 201

adsorbed phosphorus formed from the use of phosphorus fertilizers can provide adequate solution phosphorus for high crop yields on most soils. Fixation of fertilizer phosphorus results in low uptake of the fertilizer phosphorus during the year of application. Commonly, only 10 to 20 percent of phosphorus in fertilizer is used by plants during the year of fertilizer application. Therefore, repeated use of phosphorus fertilizers results in an increase in soil phosphorus content, and in many instances, soils have become sufficiently high in phosphorus that further additions of phosphorus fertilizers do not increase plant growth.

Plant Phosphorus Relations Phosphorus plays an indispensable role as a universal fuel for all biochemical activity in living FIGURE 12.10 Percentage distribution of four forms cells. High-energy adenosine triphosphate (ATP) of phosphorus as related to weathering or soil age. bonds release energy for work, when converted to (Adapted from S. C. Chang and M. L Jackson. 1958. adenosine diphosphate (ADP). Phosphorus is "Soil Phosphorus Fractions in Some Representative Soils." Jour. Soil Sci. 9:109-119. Used by also an important element in bones and teeth. The permission.) relation of phosphorus in soils to plants and animal health, and the extensive occurrence of phosphorus deficiency in grazing animals, are well known. 10 percent is supplied by mass flow and root Some of the phosphorus deficiency symptoms interception. Diffusion of phosphorus to roots is of plants are not specific. The growth of both the major limiting step in phosphorus uptake from shoots and roots is greatly reduced. One of the soils by roots. common diagnostic aids is foliage color. Phos- Obviously, the greater the phosphate concen- phorus is mobile in plants and, when a deficiency tration of the soil solution, the greater is the avail- of phosphorus occurs, phosphorus is moved from ability of soil phosphorus. The problem in agricul- the older leaves to the younger leaves at the top of ture is to maintain an adequate phosphorus the plant: Deficiency symptoms first appear on the concentration in the soil solution, even though older, or bottommost, leaves. The deficiency there is rapid fixation of phosphorus. An adequate symptom is commonly a purplish color as a re- supply of solution phosphorus can result from sult of increased anthocyanin development in the use of phosphorus fertilizers. The first fixation phosphorus-deficient tissue, as shown for corn in compounds, whether in calcareous or acid soils, Color Plate 2. tend to be amorphous, and they are more avail- Low phosphorus in animal diets is a common able than the crystalline apatite and aluminum problem. Some relationships between soil phos- and iron phosphates, which form slowly over phorus levels, phosphorus levels in plant tissue, time. The insoluble crystalline forms require years and phosphorus deficiency in dairy cattle are to form, but in the meantime, the amorphous and shown in Figure 12.11. As the amount of fertilizer 202 PLANT-SOIL MACRONUTRIENT RELATIONS

growth. Some soils are very deficient in available potassium, whereas others are very sufficient. This is in stark contrast to the general needs of nitrogen and phosphorus for high-yielding crops. Basically, potassium in soils is found in minerals that weather and release potassium ions (K+). These ions are adsorbed onto the cation exchange sites. The exchangeable potassium tends to maintain an equilibrium concentration with the soil solution from which roots absorb K + . Organic soils tend to be deficient in available potassium because they contain few minerals with potassium. FIGURE 12.11 Effect of phosphrus fertilizer application on concentration of phosphorus in oats and alfalfa. About 0.3 percent of phosphorus (dashed line) is required by dairy cattle for normal growth. Soil Potassium Cycle (From W. H. Allaway, 1975.) The earth's crust has an average potassium content of 2.6 percent. Parent materials and youthful soils could easily contain in a plow layer 40,000 to phosphorus increased (or available soil phos- 50,000 pounds per acre, or kilograms per hectare. phorus increased), the phosphorus content of oat The potassium content below the plow layer grain and alfalfa hay was markedly increased. Be- could be similar. About 95 to 99 percent of this cause cattle require about 0.3 percent phos- potassium is in the lattice of the following min- phorus in their diet for optimum growth, oat grain erals: contained sufficient phosphorus without fertilization. Oat straw remained phosphorus deficient for cattle, even with phosphorus fertilization of the oat crop. Cattle diets consisting mainly of grasses usually require a phosphorus supplement to insure adequate dietary phosphorus. The use of phosphorus fertilizer changed alfalfa hay from an inadequate phosphorus source to an adequate source. It is interesting to note that the U.S. race- Micas, especially biotite, weather faster and re- horse industry is centered on the high phos- lease their potassium much more readily than do phorus soils of Kentucky and Florida. feldspars. The feldspars tend to exist as larger Phosphorus deficiency is not a serious problem particles than micas, with the feldspars largely in in human nutrition, because of the large amount the sand and silt and the micas in silt and clay of cereal grains and meat consumed, which are fractions. good phosphorus sources. For humans, the use of Weathering of feldspar results in the dissolu- phosphorus fertilizer is more important for in- tion of the feldspar crystal and release of K+ to the creasing the quantity of food than for increasing soil solution. The potassium in micas is interlayer the phosphorus content of the food. potassium. Weathering of micas results in the migration of K+ out of the interlayer space along the edge of weathering mica particles; the weath- POTASSIUM ered edge of mica is shown in Figure 12.12. The There is a wide range in the potassium content of weathered edge of the mica particle in Figure soils and availability of potassium for plant 12.12 is vermiculite. POTASSIUM 203

The K+ appears in the soil solution or is ad- Plant removal and leaching tend to occur dur- sorbed onto a cation exchange site (step 1 in ing the spring and summer, when the supply of Figure 12.13). An equilibrium tends to be estab- available or exchangeable potassium is reduced. lished between the solution potassium and ex- In winter, by contrast, plant uptake and leaching changeable potassium (steps 2 and 3). The of potassium may be minimal and the release of amount of solution potassium is about 1 percent potassium by weathering results in an increase in as large as the exchangeable potassium. exchangeable potassium. In other words, in win- As weathering proceeds, in the absence of the ter the direction of the net effect of the reactions is removal of K+ from the soil by plant uptake or of the order 1, 2, and 4. In spring and summer, the leaching, there is a buildup of exchangeable po- direction is 6 (and 7), 3, and 5. tassium as a result of weathering. The exchange- Potassium fixation is affected by the equilib- able potassium maintains a quasi-equilibrium rium conditions in the soil and soil drying and with the potassium entrapped or fixed, as shown wetting. The application of potassium fertilizer in step 4 in Figure 12.13. Fixation is the reverse of greatly increases the amount of solution po- the weathering of mica and occurs as the amount tassium. Drying the soil at this time results in of exchangeable potassium increases and K+ increased concentration of K+ in the soil solution move back into voids where interlayer potassium at the edges of clay particles and the movement of weathered out at an earlier time. Release, the potassium into the interlayer spaces, where it be- opposite of fixation and shown as step 5, is en- comes entrapped or fixed. By contrast, soon after + couraged when plant uptake and leaching (steps the harvest of a crop, the concentration of K in 6 and 7 in Figure 12.13), result in reduced solution the soil solution is low. Drying the soil at this time potassium. A reduced concentration of solution results in the migration of potassium from inter- potassium is followed by reduced exchangeable layer spaces and increases the amount of avail- potassium, which is then followed by the release able or exchangeable potassium. of fixed potassium.

Summary Statement When plants are dor- FIGURE 12.12 Weathered mica silt particle showing mant and leaching is minimal, weathering of po- loss of potassium from the darkened area around the tassium minerals results in an increase in solu- edges. The loss of potassium results in expansion of tion potassium, followed by an increase of the 2:1 mica layers. The weathered zone is exchangeable potassium, followed by an increase vermiculite. (Photograph courtesy M. M. Mortland.) in the amount of fixed potassium. Normally, this occurs in fall and winter in minimally and moderately weathered soils. When plant uptake and leaching occur, normally in spring and summer, there is a decrease in solution potassium, followed by a decrease of exchangeable potassium, and then a decrease in the amount of fixed potassium. Weathering and subsequent loss of potassium by leaching with time can deplete the soil of potassium minerals and create soils with few potassium minerals and very low available potassium. This is generally true for intensively weathered soils. Potassium does not complex with organic com- 204 PLANT-SOIL MACRONUTRIENT RELATIONS

FIGURE 12.13 Major processes in the soil potassium cycle.

pounds; that is, it is not an integral part of any that can be released. In these soils, the solution organic molecules in plants. Therefore, po- potassium is quickly replenished when removed tassium availability is only minimally related to by roots. It would seem that potassium fixation is the soil's organic matter content in mineral soils. completely beneficial because potassium is pro- In fact, the most potassium-deficient soils are tected from leaching, yet has good potential for those composed mostly of organic matter, such plant availability in the future. However, some as organic soils. soils have an unusually large potassium-fixing capacity (vermiculitic soils), which competes with plants for potassium in fertilizer that is applied to Plant Uptake of Soil Potassium soils, and much of the applied potassium is "lost" The soil's ability to maintain a desirable concen- to fixation. tration of solution potassium is determined by the The clays in intensively weathered soils are it's potassium buffer capacity. The potassium buf- mainly kaolinite and oxidic clay. These soils con- fer capacity is the ability of the solid phase of the tain little if any fixed potassium and have low soil to buffer changes in the K+ concentration of potassium buffering capacity. the soil solution. Soils with high potassium buffer In many minimally and moderately weathered capacity tend to be fine-textured and contain a soils, the exchangeable potassium is about equal large amount of fixed potassium or interlayer po- to the needs of the crop, and the amount in solu- tassium (contain abundant mica and vermiculite) tion at any one time is equal to I to 3 percent of CALCIUM AND MAGNESIUM 205 the exchangeable. As with the case of phosphorus, the amount of potassium in the soil water is small compared with plant needs. The result is that diffusion is very important for moving potassium to roots for uptake, because mass flow cannot provide an adequate amount. For corn, it has been reported that diffusion accounts for about 80 percent of the potassium uptake. Diffu- sion is the rate-limiting step for plant uptake of potassium from most soils. Very high potassium fertilization may produce an unusually high concentration of solution potassium. Then, plants may absorb much more potassium than is needed for growth-producing luxury uptake, which is the absorption of nutrients by plants in excess of their need for growth. The luxury uptake of potassium reduces the uptake of other cations, especially magnesium, and, to a lesser extent, calcium (see Figure 12.14). FIGURE 12.14 The calcium, magnesium, and potassium content of alfalfa plants are a function of Plant Potassium Relations the potassium concentration in the nutrient solution. Potassium enhances the synthesis and translo- (Adapted from Wallace, 1948.) cation of carbohydrates, thereby encouraging cell wall thickness and stalk strength. A deficiency is sometimes expressed by stalk breakage, or lodg- als by weathering and occur as exchangeable ing. Potassium can be substituted for by sodium cations. They are the most abundant in young and in some of its physiological roles in some plants. minimally weathered soils and least abundant in The deficiency symptoms are the most severe intensively weathered and leached soils. All three on old growth, because potassium is mobile in elements are absorbed by roots as cations from plants. Potassium deficiency symptoms are quite the soil solution. characteristic on many plants as a yellowing, and Some important calcium minerals include cal- eventually death, of leaf margins. For alfalfa, cite, dolomite, gypsum, feldspar, apatite, and am- white spots first appear along leaf margins fol- phibole. Important magnesium minerals include: lowed by a coalescing of the spots to give a mar- dolomite, biotite, serpentine, hornblende, and ginal edge effect. The dying of the edges of older olivine. In contrast to potassium, there is no sig- leaves is commonly called necrosis. Potassium nificant fixation of calcium or magnesium into deficiency symptoms of corn and alfalfa are unavailable forms. The cations set free by weath- shown in Color Plate 1. ering are adsorbed onto the cation exchange sites. An equilibrium tends to be established between the exchangeable and solution CALCIUM AND MAGNESIUM forms. Compared to potassium, the amounts of There are many similarities between the behavior exchangeable and solution calcium and magne- of calcium and magnesium with those of po- sium are usually many times larger than the tassium in soils. They are all released from miner- amounts of exchangeable and solution po- 206 PLANT-SOIL MACRONUTRIENT RELATIONS

tassium. By contrast, plant needs for calcium and leaves. This causes the lower leaves to exhibit the magnesium are much less than for potassium. deficiency symptoms. The magnesium-deficiency Mass flow commonly moves more calcium and symptom on coffee leaves is shown in Color magnesium to root surfaces than plants need, Plate 4. whereas at the same time, plants need more po- Calcium is immobile in plants. When a de- tassium than is moved to surfaces surfaces. Thus, ficiency occurs, there is malformation and dis- calcium and magnesium limit plant growth only integration of the terminal portion of the plant. occasionally. In fact, calcium deficiencies in The deficiency symptom has been established plants are rare, and magnesium deficiencies oc- for many plants by the use of greenhouse meth- cur only occasionally. When deficiencies of cal- ods. As indicated earlier, calcium-deficiency cium do occur, it is usually on intensively weath- symptoms are seldom seen in fields. This is ered soils. because other serious nutrition problems usu- Magnesium is less strongly adsorbed to cation ally occur before calcium becomes sufficiently exchange sites than is calcium. The cation deficient. exchange capacity of neutral soils may be 75 percent or more calcium saturated and only 15 Soil Magnesium and Grass Tetany per cent magnesium saturated. Magnesium Grass tetany (grass staggers) is a major disease of deficiency is usually associated with acidic sandy cattle, which is characterized by muscle spasms, soils. In Michigan, acidic sandy soils with less especially in the extremities. The disease is due to than 75 pounds of exchangeable magnesium per a magnesium deficiency. The incidence of grass acre-furrow-slice are likely to be magnesium de- tetany is more common when cattle consume a ficient for corn. By contrast, soils formed from diet consisting mainly of grass rather than le- serpentinite (magnesium rich) parent material gumes. This is because the magnesium content of contain abundant magnesium and very low ex- grasses is lower than that of legumes. The inci- changeable calcium. The extreme imbalance be- dence of grass tetany is generally low when tween calcium and magnesium causes severe cal- grasses have 0.2 percent or more magnesium. The cium deficiency. Plant growth on such soils is incidence of grass tetany is related to levels of meager and the soils are called serpentine available soil magnesium, plant species, and tem- barrens. perature. Conditions that favor the incidence of grass tetany include: (1) very low exchangeable magnesium (2) reduced uptake of magnesium by Plant Calcium and Magnesium Relations plants caused by high levels of available po- Magnesium is a constituent of chlorophyll. As tassium (see Figure 12.14), sometimes caused with several other nutrients, a magnesium defi- by the application of potassium fertilizers; and ciency results in a characteristic discoloration of (3) cool, wet spring weather that encourages the leaves. Sometimes, a premature defoliation of the growth of grasses, relative to legumes, in pastures plant results. The chlorosis of tobacco, known as where the animals graze. sand drown, is due to magnesium deficiency. Cot- ton plants suffering from a lack of magnesium produce purplish-red-colored leaves with green SULFUR veins. Leaves of sorghum and corn become stripped; veins remain green with yellow intervein Sulfur exists in some soil minerals, including gyp- - strips (as shown for corn in Figure 12.15). The sum (CaSO4 2H 2 0). Mineral weathering releases 2 magnesium is mobile in plants and is moved from the sulfur as sulfate (SO4 -) which is absorbed by the older, lower leaves to the younger, upper roots and microorganisms. Sulfur is released as SULFUR 207

fertilizers used in the United States. For these reasons, sulfur is seldom limiting for plant growth even though plants require about as much sulfur as phosphorus. Sulfur deficiencies tend to occur where there is little input of sulfur by precipitation due to low precipitation and little input of sulfur into the atmosphere by burning of fossil fuels. The output of sulfur is so great near some mining and smeltering operations that nearby vegetation is killed. The addition of sulfur to soils via precipitation contributes to the development of soil acidity. Sulfate is adsorbed onto the surfaces of iron and aluminum oxides and onto the edges of silicate clays, where aluminum is located. As soil pH decreases, the capacity for sulfur adsorption increases. This is one reason why acid rain has had only a minimal effect on many soils; the more acid the soil becomes, the more sulfur is adsorbed and fixed or inactivated. Sulfur is lost from soils as sulfate by leaching and as H2S gas, which is produced by microbial reduction of sulfates in anaerobic soils. Areas in the United States in which crops have responded to sulfur are shown in Figure 12.16. Most of the locations occur where low amounts of sulfur are added to soils by precipitation and/or soil parent materials have low sulfur content. The FIGURE 12.15 Magnesium deficiency symptoms on corn. The veins are green and the intervein areas of annual addition of sulfur to soils from precipi- the lower leaves are yellow. tation was more than 200 pounds per acre near Chicago, Illinois, and only 6 pounds per acre in eastern Nebraska. Note in Figure 12.16 that no response to sulfur is shown for Illinois, whereas some response to sulfur has been obtained in sulfur dioxide (SO2 ) into the atmosphere by the eastern Nebraska. Generally, the industrial North- burning of fossil fuels and becomes an important east receives sufficient sulfur from precipitation to constitutent of the precipitation. In many loca- ensure sufficient amounts for crops. In many irri- tions, more sulfur is added to soils (via precipi- gated fields in semiarid and arid regions, enough tation) than plants need. Sulfur accumulates in sulfur is added by the irrigation water to ensure soils as organic sulfur in plant residues and is adequate sulfur for crops. 2 then mineralized to 50 4 -. The net effect is the Plants that have the greatest need for sulfur accumulation of sulfur in soils as organic sulfur. include: cabbage, turnips, cauliflower, onions, Plants, depending on the SO 2 content of the air, radishes, and asparagus. Intermediate sulfur us- may absorb sulfur through leaf stomates. In the ers are legumes, such as alfalfa and cotton and past, sulfur has been an important element in tobacco. Grasses have the lowest sulfur require- 208 PLANT-SOIL MACRONUTRIENT RELATIONS

FIGURE 12.16 Locations where crops have responded to sulfur fertilizers. (Data from Jordan and Reisenauer, 1957.)

ment. Sulfur deficiency symptoms are similar to minerals and in soil organic matter. Soil pH, by its nitrogen in that sulfur-deficient plants are stunted control of the ions in solution, greatly affects and light-green to yellow in color. phosphorus fixation and, therefore, phosphorus availability in soils. The low concentration of solution phosphorus limits phosphorus uptake by plants and results in very limited leaching of phos- SUMMARY phorus. Macronutrients are used by plants in relatively Potassium is released by mineral weathering large amounts and include nitrogen, phosphorus, and the available supply is the exchangeable potassium, calcium, magnesium, and sulfur. potassium. During periods of limited plant uptake Plant-deficiency symptoms for specific nutri- and leaching, release of potassium by weathering ents are related to the mobility of the nutrients results in increases in solution, exchangeable, within the plants. fixed potassium in most minimally and moder- Nitrogen availability in soils is controlled ately weathered soils. During periods of active mainly by mineralization and immobilization. Ni- plant uptake and leaching, there is a decrease in trogen is added to soils by fixation. and an ap- solution, exchangeable, and fixed potassium. The proximate amount is lost annually by denitri- rate of potassium diffusion to roots is the limiting fication. The dominant form of available nitrogen step in potassium uptake from soils. is nitrate, which is readily moved to roots by mass Calcium and magnesium have many parallels flow and lost from soils by leaching and denitri- with potassium in terms of the forms in soils. The fication. amounts of exchangeable calcium and magne- Phosphorus originates from apatite weathering sium usually exceed the amount of exchangeable and exists in soils in many different insoluble potassium, yet, plants use much more potassium REFERENCES 209

than calcium or magnesium. Calcium is rarely ciencies." Mich. Agr. Exp. Sta. Spec. Bull. 353, East deficient for plant growth, and such a deficiency Lansing. Scientific occurs usually only in intensively weathered soils. Delwiche, C. C. 1970. "The Nitrogen Cycle." American, September, pp. 137-146. Magnesium is sometimes deficient in plants Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. Wiley, growing on acid and sandy soils with low cation New York. exchange capacity. Hanway, J. 1960. Growth and Nutrient Uptake by Corn. Sulfur is released from sulfur minerals. Much of Iowa State University Extension Pamphlet 277. the sulfur in soils has been added by precipi- Jordan, H. V. and H. M. Reisenauer. 1957. "Sulfur and tation. Near industrial areas, more sulfur is added Soil Fertilty." USDA Yearbook Soil, Washington, D.C. to soils by precipitation than plants need. In some Juo, A.S,R. and B. G. Ellis. 1968. "Chemical and Physi- places, excessive additions of sulfur kill the vege- cal Properties of Iron and Aluminum Phosphates and tation. Their Relation to Phosphorus Availability." Soil Sci. Soc. Am. Proc. 32: 216-221. Mengel, K. and E. A. Kirby. 1982. Principles of Plant Nutrition. 3rd. ed. International Potash Institute, Allaway, W. H. 1975. "The Effect of Soils and Fertilizers Bern, Switzerland. on Human and Animal Health." USDA Agr. Informa- Mortland, M. M. 1958 "Kinetics of Potassium Release tion Bul. 378. from Biotite." Soil Sci. Soc. Am. Proc. 22:503-508. Barber, S. A. 1984. Soil Nutrient Bioavailability. Wiley, Paul, E. A. 1984. "Dynamics of Organic Matter in Soils." New York. Plant and Soil. 76:275-285. Black, C. A. 1968. Soil-Plant Relations. Wiley, New Stevenson, F. J. 1986. Cycles of Soil. Wiley, New York. York. Wallace, A. et al. 1948. "Further Evidence Supporting Chang, S. C. and M. L. Jackson. 1958. "Soil Phosphorus Cation-Equivalent Constancy in Alfalfa." J. A. Soc. Fractions in Some Representative Soils." Jour. Soil Agron. 40:80-87. Sci. 9:109-119. Weber, C. R. 1966. "Nodulating and Nonnodulating Iso- Cook, R. L. and C. E. Millar. 1953. "Plant Nutrient Defi- lines." Agron. Jour. 58:43-46. CHAPTER 13

MICRONUTRIENTS AND TOXIC ELEMENTS

Micronutrients are required in very small amounts humid regions, many calcareous soils exist on and function largely in plant-enzyme systems (see recent lake plains and on exposed subsoil or par- Table 13.1). The factors that determine the ent materials. At building sites, such as home- amounts of micronutrients available to plants are sites, acid soil may be contaminated with enough closely related to soil conditions and plant spe- mortar from brick wall construction, or the sur- cies. For example, a change in soil pH can change face soils may be mixed with highly calcareous a deficient situation for plants into a toxic situa- parent material during excavation and become tion. Four of the seven micronutrients are used by alkaline. Shrubs and flowers grown on these loca- plants as cations, and they will be considered tions commonly have deficiency symptoms for first. iron and/or manganese (see Figure 13.1). Cereals and grasses, including sugarcane, tend to have a manganese deficiency when grown on alkaline soils. Plants particularly susceptible to IRON AND MANGANESE iron deficiency include: roses, pin oaks, azaleas, rhododendrons, and many fruits and orna- Iron and manganese are weathered from minerals mentals. and appear as divalent cations in solution; as Deficiency symptoms of iron and manganese such, they are available2+ to plants.2+ Generally, in are striking and, for some plants, are very similar. acid soils, sufficient Fe and Mn exist in the In some cases, it is impossible to know whether soil solution to meet plant needs. In some very the symptom is due to an iron deficiency or a acid soils, manganese, and to a lesser extent iron, manganese deficiency unless trials are conducted are toxic because of the high amounts in solution. to determine which element alleviates the Deficiencies are common in alkaline soils where symptoms. In other cases, it is known from experi- oxidized forms of iron and manganese exist as ence which element is deficient when the defi- insoluble oxides and hydroxides. As a conse- ciency symptoms occur. Typical iron-deficiency quence, deficiencies are common in arid regions symptoms or iron-chlorosis appears on leaves as where many soils are calcareous and alkaline. In yellow interveinal tissue and dark-green-colored

210 IRON AND MANGANESE 211

TABLE 13.1 Essential Micronutrient Elements and Role in Plants

veins, as shown for pin oak and roses in Color Plant "Strategies" for Iron Uptake Plate 2. The younger leaves tend to be most af- Iron is fourth in abundance in the earth's crust, fected because iron is immobile in plants. following oxygen, silicon, and aluminum. Yet, The absence of sufficient manganese stunts to- iron deficiencies in plants are common because matoes, beans, oats, tobacco, and various other the amount of soluble iron in the soil solution is plants. Associated with this stunting is a chlorosis frequently too low to meet plant needs. Plants of the upper leaves. The interveinal tissue turns have developed special "strategies" for coping yellow, whereas the veins remain dark-green- with this situation in alkaline soils where the oxi- colored, as shown for kidney beans in Color Plate dized or ferric form of iron has very low solubility. 3. The gray-speck disease of oats is attributed Plants increase their ability to absorb iron from to manganese deficiency and appears as gray- calcareous and highly aklaline soils in two ways. colored areas on stems. For many nutrients, First, plant roots decrease the pH in the rhizo- varietal or cultivar differences result in differing sphere by the excretion of H+ that solubilizes plant responses to the same level of available nu- ferric iron. Second, for monocots (grasses), trients. This is illustrated by varietal response to siderophores are excreted by the roots. Sidero- manganese by soybeans as shown in Color phores (iron bodies) are metabolites secreted by Plate 4. organisms that form a highly stable coordination Many chlorotic plants growing on alkaline soils compound (organic chelate) with iron. The side- have an iron deficiency and many have a manga- rophores solublize ferric iron, which is subse- nese deficiency. Zinc may also be a contributing quently absorbed by the roots and the iron used factor. Healthy green pin oak trees were found to by the plant. Microorganisms also excrete sidero- need 55 centimeters of acid soil when growing in phores. These mechanisms enable certain plant an area where many soils were alkaline and the species to satisfy their iron needs when growing trees were chlorotic (see Figure 13.2). on alkaline soils. Many plants, however, are not 212 MICRONUTRIENTS AND TOXIC ELEMENTS

FIGURE 13.1 Iron-deficient symptoms frequently occur on isolated pin oak trees growing in areas where soils are normally acid. Because of local contamination of the soil with alkaline materials from road and building construction, the soil is alkaline. Note die-back at the top of the tree. Chlorotic leaves are yellow with green-colored veins. FIGURE 13.2 Soil reaction (pH) profiles associated with untreated green and chlorotic pin oak trees. Pin efficient in utilizing ferric iron and develop defi- oak trees were healthy if the upper 55 centimeters of ciency symptoms or fail to grow. soil was acid. (Data from Messenger, 1983.)

complexed with organic matter. As a result, cop- COPPER AND ZINC per is quite immobile in soils and the copper Copper and zinc are released from mineral weath- concentration of soil solutions tends to be very 21. ering to the soil solution as Cue' and Zn These low. Strongly adsorbed and nonexchangeable micronutrient cations can be adsorbed onto copper equilibrate with Cu e+ in solution, and 2+ cation exchange sites. Little Cu exists as ex- much of the copper in solution is complexed with changeable copper, however, but tends to be soluble organic matter of low molecular weight. strongly adsorbed to the inorganic fraction or Even though there is a very low concentration of COPPER AND ZINC 21 3 copper in solution, plants require very little cop- tillering time. The tips become white and the per, which is generally obtained in sufficient leaves appear narrow and twisted. Growth of the quantity by root interception and mass flow. Cop- internodes is depressed. In fruit trees, gum pock- per deficiencies in plants tend to occur on newly ets under the bark and twig dieback occur. developed organic soils and leached sandy soils Zinc deficiency was first recognized in plants that are characterized by low amounts of total growing on organic soils in Florida. Zinc defi- copper. Copper is not easily leached and, after ciencies occur similarly to copper on organic about 20 to 40 pounds of copper per acre have soils and leached acid sandy soils from which been applied to deficient soils, little additional zinc has been leached. Zinc is commonly defi- copper is needed. cient on alkaline and/or calcareous soils. An ex- Zinc, as Zn 2 +, occurs as an exchangeable treme case of zinc deficiency of navy beans on cation, is strongly absorbed onto several soil con- calcareous soil is shown in Color Plate 3. Applica- stitutents, and is complexed by organic matter. tion of phosphorus fertilizers at high rates has There is considerable similarity in the forms of also been found to reduce zinc availability in zinc and copper in soils and plant uptake relation- soils, especially when levels of soil zinc are only ships. Zinc, however, appears to be complexed marginally sufficient (see Figure 13.3). with organic matter to a lesser degree and has Zinc deficiencies are widespread in the United sharply reduced availability with increasing States on corn, sorghum, citrus and other fruits, soil pH. beans, vegetable crops, and ornamentals. Pecan rosette, the yellows of walnut trees, the mottle-leaf of citrus, the little-leaf of the stone fruits and Plant Copper and Zinc Relations grapes, the white-bud of corn, and the bronzing of Copper is quite immobile in plants and deficiency tung tree leaves are all due to zinc deficiency. In symptoms are highly variable among plants. In tobacco plants, a zinc deficiency is characterized cereals the deficiency shows first in the leaf tips at by a spotting of the lower leaves.

FIGURE 13.3 Navy bean plants growing on soils low in available zinc grew less well with increasing amount of applied phosphorus (zero to 720 pounds per acre) fertilizer owing to reduced availability of zinc to the plants. 21 4 MICRONUTRIENTS AND TOXIC ELEMENTS

BORON MOLYBDENUM The most abundant boron mineral in soils is tour- Weathering of minerals releases molybdenum maline, a borosilicate. The boron is released by that is adsorbed to various soil constituents. The weathering and occurs in the soil solution mostly adsorbed molybdenum maintains an equilibirum as undissociated boric acid, H 3 B0 3 . The boron with molybdenum in solution. The solution form 2- in solution tends to equilibrate with adsorbed is mainly as the molybdate ion, M004 . Molyb- boron. denum also accumulates in soil organic matter. Boron is leached from acid sandy soils, and this In acid soils, molybdenum is strongly adsorbed results in low availability. Boron availability is or fixed by iron oxides. In many instances, molyb- reduced by increasing pH due to strong boron denum deficiencies are corrected by liming, ow- adsorption to mineral surfaces (fixation). Thus, ing to increased molybdenum availability. The boron availability is a maximum in soils with in- data in Figure 13.4 show that increasing soil pH by termediate pH. Boron deficiency tends to occur in liming increased the molybdenum content of cau- dry weather; which is likely because of reduced liflower leaves. Some calcareous soils in western movement of boron to roots by mass flow in United States developed from high molybdenum water. content parent materials and have very high levels Plants absorb boron as uncharged boric acid, of available molybdenum. The plants, however, and it appears to be a simple diffusion of the are not injured by the high molybdenum level. uncharged molecules into roots. Once the boron is stablized in the plant, it is quite immobile. Apical growing points and new plant leaves become chlorotic on boron-deficient plants, and FIGURE 13.4 Increased soil pH due to liming growing points frequently die. Many physiological increased the availability of soil molybdenum and diseases of plants, such as the internal cork of resulted in an increased concentration of apples, yellows of alfalfa, top rot of tobacco, and molybdenum in cauliflower leaves. (Data from Turner cracked stem of celery are caused by a boron and McCall, 1957.) deficiency. Boron deficiency of sugar beets causes heart rot of the beet, as shown in Color Plate 3.

CHLORINE The chlorine requirement of plants is very small, even though chlorine may be one of the most abundant anions in plants. Chlorine is unique in that there is little probability that it will ever be deficient in plants growing on soils. Chlorine enters the atmosphere from ocean spray, is distributed around the world, and is added to soils via precipitation. The presence of chlorine in fertilizer, as potassium chloride, is an additional source of chlorine for many agricultural soils. MOLYBDENUM 21 5

Plant and Animal num needed for animal consumption have been Molybdenum Relations found growing on certain acid soils. The major Molybdenum is needed for nitrogen fixation in nutritional problem is molybdenum toxicity, mo- legumes. When molybdenum is deficient, le- lybdenosis, which develops in grazing animals gumes show symptoms similar to those of nitro- when forage has over 10 to 20 ppm molybdenum. gen deficiency. In cauliflower, molybdenum de- The molybdenosis problem arises because for- ficiency results in cupping of leaf edges. It is age plants have a wide range of tolerance for caused by a reduced rate of cell expansion near molybdenum, whereas animals do not. Legumes the leaf margin, compared with that in the center accumulate more molybdenum than do common of the leaf. Leaves also tend to be long and slen- grasses, and they take up more molybdenum in der, giving rise to the symptom called whiptail. wet than in dry soils. The data in Figure 13.5 show Interveinal chlorosis, stunting of plants, and gen- that large increases in molybdenum (and manga- eral paleness are also exhibited, depending on nese) can occur in plants with large increases in the kind of plant. molybdenum (and manganese) in the growing Plants and animals need very small amounts of medium. By contrast, note the virtual lack of an molybdenum. Plants deficient in the molybde- increase in iron content of the plant with an in-

FIGURE 13.5 The iron (and copper) content of tomato leaflets was little affected by a large increase in the supply of nutrient in the growing medium. The molybdenum and manganese content of leaflets was greatly increased. (From Beeson, 1955.) 21 6 MICRONUTRIENTS AND TOXIC ELEMENTS creasing amount of available iron. For soils with stomachs of ruminants incorporate cobalt into very high available molybdenum, there is no ef- vitamin B 12 , and this vitamin provides these fective method for reducing molybdenum uptake animals with cobalt. Single-stomached animals, from soils by plants. including humans, must obtain their cobalt from Most problem areas of molybdenosis are re- foods containing vitamin B12, such as milk or lated to the geologic origin and wetness of soils. meat products. Only persons who eat strictly veg- Common sources of molybdenum in the western etarian foods are likely to have a cobalt-deficient United States are granite, shale, and fine-grained diet. Cattle and sheep that are not fed legumes sandstone. Soils producing forages with high mo- usually need supplementary cobalt. lybdenum level are generally confined to valleys Cobalt deficiency of animals was observed by of small mountains streams (western United the earliest settlers in parts of New England. In States). Only a very small part of any valley actu- New Hampshire, it was called Chocorua's Curse ally produces high molybdenum forages. These and Albany Ail, from local place names. In south- soils are wet, or poorly drained, alkaline, and high ern Massachusetts, it was known as neck's ail in organic matter. Molybdenosis in Nevada and because the disease was most common on necks California is associated with soils formed from the of land that projected into bays. It is now known high-molybdenum-content granite of the Sierra that the cobalt deficiency in New England had its Nevada Mountains. A typical location where high origin in the geologic history of the soils. The molybdenum content forage is produced is low-cobalt soils formed in glacial deposits de- shown in Figure 13.6. rived from the granite of the White Mountains, which contained very little cobalt. The soils are also sandy and tend to lose cobalt by leaching. The location of low-cobalt soils in New England is COBALT shown in Figure 13.7. Cobalt is required by microorganisms that sym- Cobalt deficiency among grazing animals is a biotically fix nitrogen. This is the only known problem on the wet, sandy soils of the lower need for cobalt by plants. Microorganisms in the coastal plain in the southeastern United States.

FIGURE 13.6 Plants containing toxic levels of molydbenum are found only on wet soils fromed from high-molybdenum parent materials, area C, in this mountain valley in Nevada. Areas A and B have well-drained soils in which the molybdenum is not readily available to plants. Area D is wet, but has soils formed low- molybdenum parent materials. (Data from Allaway, 1975.) POTENTIALLY TOXIC ELEMENTS FROM POLLUTION 217

the United States have soils so low in selenium that forages for livestock are deficient in selenium. Acid soils formed from low-selenium parent materials produce forages that are selenium deficient for good animal nutrition. In the western states, a selenium toxicity is a problem associated with many soils. In 1934, mysterious livestock maladies on certain farms and ranches of the Plains and Rocky Mountain states were discovered to be caused by plants with high selenium content, which were toxic to grazing animals. Affected animals had sore feet, excessive shedding, and some died. During the next 20 years, scientists found that the high levels of selenium occurred only in soils derived from certain geologic formations with a high selenium content. Another important discovery was that a group of plants, selenium accumulators, had an extraordinary ability to extract selenium from the soil. Selenium accumulators contained about 50 ppm of selenium compared with less than 5 ppm in nonaccumulator plants. One selenium accumulator is a palatable legume, Astragalus bisulcatus. This plant was found to FIGURE 13.7 Low cobalt areas (shaded) in New England. The arrows indicate the direction of glacial have a selenium content that was about 140 times ice movement from the White Mountains containing more than that of adjacent plants. The adequacy low-cobalt rocks. (Data from Kubota, 1965.) of selenium in forages for farm animals in the United States is shown in Figure 13.8.

The low cobalt soils are very sandy, poorly drained, and have humic or organic pans (Bh POTENTIALLY TOXIC ELEMENTS horizons). The original sandy parent materials FROM POLLUTION had little cobalt, and the cobalt was leached from the upper soil horizons during soil development. Potential poisoning of humans by arsenic, ca- These soils have the lowest cobalt content of any dimum, lead, and mercury are real concerns to- soils in United States. Consequently, little cobalt day. Arsenic has accumulated in soils from sprays is available to forage plants growing on these used to control insects and weeds and to defoliate soils. crops before harvesting. Although arsenic accumulates in soils and has injured crops, it has not created a hazard for humans or animals. Cadmium poisoning has occurred in Japan SELENIUM from the dumping of mine waste into rivers, where Selenium (Se) is not required by plants but is fish ingested the cadmium from the mine waste. needed in small amounts by warm-blooded Cadmium also appears in some sewage sludges. animals and, probably, humans. Large areas of Using soils for the disposal of sewage represents a 21 8 MICRONUTRIENTS AND TOXIC ELEMENTS

FIGURE 13.8 Areas where forages and feed crops contain various levels of selenium. (Data from Allaway, 1975.) potential danger. Mine spoils may have toxic lev- resents an important limitation in the use of soils els of cadmium, but natural agricultural soils do for sewage and industrial waste disposal. Sewage not contain harmful levels of cadmium. known to be lacking in heavy metals can be added Lead is discharged into the air from automobile to soils without any danger of heavy metal poi- exhausts and other sources, and it eventually soning of humans or other animals. reaches the soil. In soil, lead is converted to forms unavailable to plants. Any lead that is absorbed tends to remain in plant roots and is not trans- RADIOACTIVE ELEMENTS ported to the shoots. Soils must become very pol- luted with lead before significant amounts move Rocks and soils naturally contain radioactive ele- into the tops of plants. ments. Currently, there is concern about radon. Mercury is discharged into the air and water Many radioactive elements have very short half- from use in pesticides and industrial activities. lives, and contamination of the soil with these Under conditions of poor aeration, inorganic mer- elements is of little concern. Radioactive cesium cury is converted to methyl mercury, which is very (Cs) is produced by atomic bombs and has a long toxic. Plants do not take up mercury readily from half-life. Radioactive cesium appears to be fixed soils; however, soils should not be used to disin vermiculitic minerals much like potassium. pose of mercury because of the highly toxic na- This limits its availability to plants. The soil tends ture of methyl mercury. to slow down the movement of radioactive ce- The soil is being used increasingly for sewage sium from the soil into plants and, later, into disposal. The application of high levels of heavy animals. metals, and their potential uptake by plants, rep- The contamination of soils is via the atmo- REFERENCES 219

sphere, and radioactive elements that fall on vege- boric acid. Boron is leached from acid mineral tation are absorbed by the leaves. Testing of nu- soils, resulting in plant deficiency. In alkaline clear weapons prior to 1962 resulted in significant soils, boron is fixed. contamination of tundra vegetation in Alaska. In- Chlorine is picked up by winds from seawater creased radioactivity was found in the caribou and distributed via precipitation throughout the that grazed on the tundra. People whose major world. Chlorine deficiency in plants growing on food was caribou were found to have about 100 soils is highly unlikely. times more radioactivity in their bodies than peo- Molybdenum availability is greatly affected by ple in the continental United States. More re- soil pH. In acid soils molybdenum is fixed by iron, cently, a similar contamination occured in north- and plant deficiencies occur. If some of these ern Europe as a result of the Chernobyl nuclear soils are limed and the pH is increased, molybde- reactor explosion in the Soviet Union. num may become toxic for plants. Molybdenum Nuclear bomb tests were conducted on Bikini toxicity for grazing animals is related to soil pH, Atoll of the Marshall Islands between 1946 and soil organic matter content, and soil wettness. 1958. The soils were heavily contaminated with Cobalt is needed by microrganisms that fix ni- radioactive cesium-137. Several approaches have trogen. been suggested to decontaminate the soil. First, Selenium is not needed by plants but is needed mix a large quantity of potassium with soil and for animals. The selenium content of forages is irrigate it with seawater to increase the likelihood highly variable, depending on soil, producing for- of potassium uptake by plants relative to cesium. ages for farm animals that range from deficient to A second approach is to remove the top 16 inches toxic. of topsoil and replace it with uncontaminated Potentially toxic elements sometimes added to soil. The Bikinians favor the second approach. soils include arsenic, cadimum, lead, and mercury. Plants and soils receive readioactive elements via the precipitation or fall-out from the atmo- SUMMARY sphere. Micronutrients are needed by plants in very small amounts and function largely in plant-enzyme systems. The reduced (divalent) forms of iron and man- REFERENCES ganese are available to plants. Plant deficiencies Allaway, W. H. 1975. "The Effects of Soils and Fertilizers are associated with the oxidized forms, especially onHuman and Animal Health." USDA Agr. Inform. in alkaline soils. Toxic concentrations may occur Bull. 378, Washington, D.C. in very acid soils. Plants have developed special Anonymous. Fallout in the Food Chain. Time, Septem- ways to obtain iron from alkaline soils, including ber 13, 1963. the excretion of hydrogen ions and siderophores. Beeson, K. C. 1955. "The Effects of Fertilizers on the Copper and zinc are released to the soil solu- Nutritional Quality of Crops," in Nutrition of Plants, tion by mineral weathering. Uptake is mainly as Animals, and Man Symposium Proc. Michigan State University, East Lansing. divalent cations. Copper deficiencies tend to oc- Bienfait, H. F. 1988. "Mechanisms in Fe-Deficient Reac- cur on acid sandy soils and organic soils. Plant tions of Higher Plants." J. Plant Nut. 11(6-11): deficiencies of zinc are common on alkaline min- 605-629. eral soils and organic soils. Chaney, R. L. 1988. "Recent Progress and Needed Re- Boron is released by the weathering of tourma- search in Plant Iron Nutrition." J. Plant Nut. 11(6- line, and plant uptake is mostly as undissociated 11):1589-1603. 220 MICRONUTRIENTS AND TOXIC ELEMENTS

Elliott, S. 1988. "Bikini Still Atomic No-man's Land." Lewin, R. 1984. "How Microorganisms Transport Iron." Lansing State Journal, May 22, 1988. Science. 225:401-402. Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. Wiley, Messenger, A. S. 1983. "Soil pH and the Foliar Macro- New York. nutrient/Micronutrient Balance of Green and Inter- Kubota, J. 1965. "Soils and Animal Nutrition." Soil Con. veinally Chlorotic Pin Oaks." J. Environ. Hort. 1(4): November, pp. 77-78. 99-104. Kubota, J. 1969. "Trace Element Studies Link Animal Turner, F. and W. W. McCall. 1957. "Studies on Crop Ailments with Soils." Soil Conservation. Vol. 35, No- Response to Molybdenum and Lime in Michigan." vember, pp. 86-88. Mich. Agr. Exp. Sta. Quart. Bul. 40:268-281. CHAPTER 14

FERTILIZERS

Manures and other materials have been applied to manufacturing methods and have a uniform com- soils for thousands of years to increase plant position. growth. About 150 years ago, some of the essential plant nutrients were discovered and, subsequently, fertilizers containing essential plant nu- Grade and Ratio trients were manufactured and applied to soils to The fertilizer grade expresses the nutrient content increase fertility. By 1855, researchers at the Rot- of fertilizers. The grade is the percentage compo- hamsted Agricultural Experiment Station, near sition, expressed in the order: N-P 20 5-K2 0. The London, England, declared that soil fertility could grade, 27-3-9, contains 27 percent nitrogen, phos- be maintained permanently with mineral fertil- phorus equivalent to 3 percent P2 05 , and po- izers. Today, fertilizer use has been estimated to tassium equivalent to 9 percent K 20. Most of the account for 20 to 25 percent, and 30 to 40 percent, nitrogen and potassium in fertilizers is water solu- of the total crop production in the world and ble. In the United States the phosphorus in the United States, respectively. The close parallel be- fertilizer must be sufficiently soluble to dissolve in tween the yields of corn (maize) and the applica- neutral ammonium citrate. Other countries have tion of nitrogen fertilizer in Illinois is shown in different laws regarding the solubility or availabil- Figure 14.1. ity of the nutrients in fertilizers. Comparisons between fertilizers can be made only on the basis of their nutrient content, not according to the kinds of compounds in the fertilizer. FERTILIZER TERMINOLOGY The Soil Science Society of America favors a A fertilizer is any organic or inorganic material of fertilizer grade based on the percentages of nitro- natural or synthetic origin (other than liming ma- gen, phosphorus, and potassium. In some cases, terials) that is added to soils to supply one or the fertilizer composition is expressed on this more essential nutrient elements. Most of the fer- basis. tilizers considered in this chapter are inorganic The fertilizer ratio is the relative proportions of materials that are the product of sophisticated primary nutrients in a fertilizer grade divided by

221 222 FERTILIZERS

comes the guaranteed analysis; and (3) name and address of manufacturer. Fertilizers that are offered for sale may be sampled by inspectors at any time, and analyzed to determine whether the fertilizers meet the guaranteed analysis. The inspection and analysis may be under the control of the Department of Agricul- ture, the Agricultural Experiment Station, or a state chemist. In this way purchasers are assured that manufacturers comply with the fertilizer laws.

Types of Fertilizers Fertilizers that contain only one compound and supply one or two plant nutrients are called fertilizer materials or carriers. Fertilizer carriers or materials that are mixed together and processed to produce fertilizers containing at least two or more nutrient elements are mixed fertilizers. Fertilizer may be in the form of a solid, liquid, or gas.

FERTILIZER MATERIALS The first fertilizer material was manufactured FIGURE 14.1 Corn yields and nitrogen fertilizer use when ground bones were treated with sulfuric n Illinois; 5-year averages. (Data from Welch, 1986.) i acid to increase the solubility (availability) of the phosphorus. This occurred in England in about 1830, and was the beginning of the modern fertil- the highest denominator for that grade. A 27-3-9 izer industry. This was followed by the develop- fertilizer has a ratio of 9-1-3. This fertilizer is high ment of potassium materials from dried sea de- in nitrogen, low in phosphorus, and moderate in posits late in the nineteenth century, and the potassium, relatively speaking. Such a fertilizer manufacture of nitrogen materials from the fixa- provides good maintenance for lawns in which tion of atmospheric in the early twentieth the soil phosphorus level from past fertilizer use is N2 century. high and the major need is for nitrogen.

Nitrogen Materials General Nature of Fertilizer Laws The source of essentially all industrial nitrogen, In general, the laws controlling the sales of fertil- including fertilizer nitrogen, results from the fixa- izers in various states are similar. They all require tion of atmospheric N , according to the following periodic registration of the brands or grades and 2 generalized reaction: accurate labeling. Most states require that the following information be made available to purchasers: (1) name or brand; (2) grade, which be- FERTILIZER MATERIALS 223

The hydrogen is usually obtained from natural gas ure 14.3. Oxidation of NH 3 to nitric acid and neu- (CH4) and the overall reaction, using 8N 2 and 20 2 tralization of the HN03 with NH3 produces ammo- to represent air, is nium nitrate (NH4NO3 ). The CO2 from NH 3 manufacture is used to produce urea [CO(NH2)2]:

The NH 3 is ammonia, commonly called anhydrous ammonia. Much of the ammonia is pro- Urea [CO(NH2)2] hydrolyzes in soil in the duced in the southern states near petroleum and presence of urease to form ammonia. The appli- natural gas fields and is transported by pipeline to cation of urea, in effect, is the same as applying various parts of the United States. Being 82 per- ammonia. The placement of urea on the soil sur- cent nitrogen, ammonia is the most concentrated face (as for lawn fertilization) results in the signif- nitrogen material and the least expensive on a per icant loss of NH 3 to the atmosphere by volatiliza- pound of nitrogen basis. Low cost has contributed tion unless rain or irrigation leach the urea into to its widespread use. the soil. Urea and ammonium nitrate should not At normal temperature and pressure, ammonia be mixed or stored together, because the mix is a gas, but it is stored and transported under adsorbs much water from the air and forms an sufficient pressure to keep it a liquid. Locally, unfavorable physical condition. ammonia is handled by tank trucks and trailers for Reaction of NH3 with (1) sulfuric acid produces direct application to fields, as shown in Figure ammonium sulfate, (2) phosphoric acid produces 14.2. When the ammonia is released from injec- ammonium phosphate, and (3) water and ammo- tors at a depth of about 15 centimeters and reacts nium nitrate produce nitrogen solutions (see Fig-

with the water of moist soil, NH4' is formed, ure 14.3). All of these nitrogen materials are water which is adsorbed onto cation exchange sites. soluble. However, application of ammonia at shallow In general, water-soluble nitrogen materials are depths in dry soil can result in considerable loss about equally effective when they are used on the of ammonia by direct escape from the soil as basis of an equivalent amount of nitrogen and are a gas. placed within the soil. For ammonium nitrate About 98 percent of the nitrogen fertilizer pro- (NH4NO 3), dissolution into the soil solution pro- - duced in the world is ammonia or one of its deriv- duces both N0 3 and NH4' for plant uptake. Un- atives. The manufacture of ammonia and five of der conditions of well-aerated soil and a pH 6 or its derivatives is shown diagrammatically in Fig- above, NH4' is quickly nitrified to N03. The result is that it often makes little difference, whether the nitrate or ammonium form of nitro- FIGURE 14.2 Injection of anhydrous ammonia into gen is applied. A few exceptions include the use soil. The ammonia is handled as a liquid under of ammonium for flooded rice production to pressure. avoid denitrification and the strong acid-forming effect of ammonium sulfate. For this reason, (NH 4) 2SO4 is preferred if an increase in soil acidity is beneficial. The application of a soluble nitrogen fertilizer typically results in large, rapid increases in the nitrate concentration of the soil solution. Under these conditions, plant uptake of nitrogen is rapid and the fertilizer nitrogen is quickly depleted. An application of nitrogen fertilizer on a lawn in May 224 FERTILIZERS

FIGURE 14.3 A diagrammatic representation of the manufacture of ammonia and some of its fertilizer derivatives. may result in little, if any, fertilizer nitrogen left in Phosphorus Materials June. Consequently, there is a need for a less The starting point for the manufacture of phos- soluble nitrogen fertilizer. Urea-formaldehyde phorus fertilizers is phosphate rock, which is a and sulfur-coated urea are insoluble or slow- calcium fluorophosphate of sedimentary or igne- release fertilizers designed to supply nitrogen ous origin. Huge deposits of phosphate rock, also slowly and reduce the frequency of nitrogen appli- called rock phosphate, are located east of Tampa, cation. These fertilizers are more expensive and Florida, where about 70 percent of the phosphate their use is restricted to lawns, golf courses, and ore in the United States is mined. The sedimentary horticulture. deposits were formed 10 to 15 million years ago A brief summary of some properties of nitrogen and are buried under 5 to 10 meters of sand. materials is given in Table 14.1. Mining consists of removing the sandy overbur-

TABLE 14.1 The Principal Fertilizers Supplying Nitrogen FERTILIZER MATERIALS 225 den and using a hydraulic gun to break up the ore superphosphate, as is shown by the following to form a slurry, as shown in Figure 14.4. The equation: slurry is pumped to a recovery plant, where rock phosphate pebbles are separated from sand and clay by a washing and screening process. The rock phosphate pebbles are ground to a powder, which is sometimes used directly as a fertilizer. The primary phosphorus mineral in phosphate The ordinary superphosphate produced in the rock is fluoroapatite, Ca 1O F2 (PO4)6. Apatite is preceding reaction consists of about one half also the source of phosphorus in most soil parent monocalcium phosphate and one half calcium materials. Apatite is quite insoluble, and when sulfate. Hydrogen fluoride is one of the most haz- phosphate rock is applied to soil, very little phos- ardous pollutants if released into the atmosphere. phorus dissolves. Its use is restricted to areas The gases from manufacture of the phosphorus close to the mines, where it is inexpensive, or to fertilizer are passed through a scrubber and the its application in very acid soil, where soil acids fluorine is recovered. Some of the fluorine is used promote dissolution. Rock phosphate is a pre- to fluorinate water. Ordinary superphosphate is ferred source of phosphorus by some organic about 9 percent phosphorus, which equals 20 farmers. percent P2 05 equivalent and has a grade of 0-20-0. As with ground bones, acidulation of rock The phosphorus in monocalcium phosphate is phosphate increases solubility and plant avail- water soluble. Very little 0-20-0 is used today, ability of the phosphorus. The acidulation of rock because other phosphorus fertilizers are less ex- phosphate with sulfuric acid produces ordinary pensive on an equivalent phosphorus basis.

FIGURE 14.4 Mining rock phosphate. The overburden is ,removed and a hydraulic gun is used to break up the ore layer and form a slurry that is pumped to a recovery plant. (Photograph courtesy International Minerals and Chemical Corporation.) 226 FERTILIZERS

Under proper conditions, the reaction of rock In the washing process to remove rock phos- phosphate with excess sulfuric acid will produce phate pebbles, the very small rock phosphate phosphoric acid (H3 PO 4). Treatment of phos- particles (many colloidal-sized) are put into waste phate rock with phosphoric acid produces super- ponds. Here, the particles settle out and the water phosphate with a higher content of phosphorus. evaporates. These low-soluble and low-value ma- The reaction is: terials are processed and marketed under various names, including colloidal phosphate. Bonemeal is also a phosphorus source, but it is too expen- The same water-soluble monocalcium phosphate sive to use on farm fields. Both of these materials is produced, but without the CaSO 4 . The grade is are natural products and are preferred by some 0-45-0 and has been called concentrated, or triple gardeners. A summary of major materials contain- superphosphate. Early production of superphos- ing phosphorus, including their phosphorus con- phate was mainly with sulfuric acid, and its use tent and some remarks, is given in Table 14.2. added a significant amount of sulfur to soils. Now, the extensive use of 0-45-0 has reduced sulfur additions to soils. Ammonium phosphates are produced by neu- Poaim Maeial tralizing phosphoric acid with ammonia. Two Wood ashes contain several percent potassium popular kinds are produced, depending on the and are an important source of this element. Po- extent of neutralization-monoammonium phos- tassium mines were opened in Germany in the phate (MAP), 11-48-0, and diammonium phos- late nineteenth century and at Carlsbad, New phate (DAP), 18-46-0. The actual grade is variable, Mexico, in the early twentieth century. Mining of depending on the proportions of the reactants. the hugh potassium deposits in Saskatchewan, Ammonium polyphosphate (APP), 10-34-0, is Canada, began in 1959 (see Figure 14.6.). These manufactured by reacting NH3 and H3 PO4 in a potassium deposits, which were formed when an- cross-pipe reactor. These phosphates contain cient seas evaporated, are called evaporate de- both nitrogen and phosphorus in water-soluble posits. The salts in the ocean water precipitated form. Routes for the manufacture of the major as the water evaporated and the salts in the water phosphorus materials are shown in Figure 14.5. became more concentrated. Later, these salt de-

FIGURE 14.5 Routes for the manufacture of phosphorus materials. FERTILIZER MATERIALS 227

TABLE 14.2 The Principal Phosphatic Materials

° Total phosphoric acid instead of amount that is available.

posits were buried under various kinds of over- Some of the Canadian deposits are relatively pure burdens and rocks. KCI and require little, if any, processing. The minerals in the deposits are representative The most popular method of processing po- of the salts in seawater, mainly salts of sodium tassium ores is flotation. The ore is ground, sus- and potassium. Slie i KCI and linie i a pended in water, and treated with a flotation agent mixture of KCI and NaCI. Langbeinie i a mixture that adheres to the KCI crystals. As air is passed of potassium and magnesium sulfates. Process- through the suspension, KCl crystals float to the ing of most of the ores consists of separating KCI top and are skimmed off. The material is dried and from the other compounds in the ore; more than screened to obtain the proper particle size. The 95 percent of the potassium in fertilizers is KCI. material, KCI, is marketed as 0-0-60.

FIGURE 14.6 A continuous mining machine at work about 1 kilometer underground in potash mine at Esterhazy, Saskatchewan, Canada. (Photograph courtesy American Potash Institute and International Minerals and Chemical Corporation.) 228 FERTILIZERS

Some KCI is obtained from brine lakes at The formula is a recipe for making the fertilizer. Searles Lake in California, and at the Great Salt The mixing together of 364 pounds of NH 4 NO3 Lake in Utah. Minor potassium materials include (33-0-0), 1,200 pounds of ordinary superphos- K2 S04 and KN0 3 . All of the potassium materials phate (0-20-0), and 400 pounds of KCI (0-0-60), contain water-soluble potassium. plus 36 pounds of conditioner, would produce 2,000 pounds of fertilizer with a grade of 6-12-12. The major systems for mixing fertilizers are granu- Micronutrient Materials lation, bulk blending, and fluid. Although the bulk of fertilizer consists of the materials containing nitrogen, phosphorus, and potassium, micronutrients are sometimes added to Granular Fertilizers fertilizer to supply one or more of the micronu- In the manufacture of granular fertilizers, dry ma- trients. In these instances, the percentage of the terials are mixed together with a little water, or dry element in the fertilizer becomes a part of the materials are mixed with acids and other fertilizer guaranteed analysis. Some common micronu- solutions to form a thick slurry. The mixing occurs trient fertilizers are listed in Table 14.3. in a heated, revolving drum where particles of different size are formed as the product dries, producing granules. Materials like diatomaceous earth (a powdery siliceous material formed from MIXED FERTILIZERS the remains of diatoms) or kaolinitic clay are Large quantities of materials containing only one added to produce a coating on the granules that or two nutrients are applied directly to the land. inhibits adsorption of water from the air and

This is particularly true for NH3, which is handled prevents caking (agglomeration or lumping). Cak- as a liquid under pressure and applied with spe- ing is due to soft granules that coalesce at contact cial applicators. Because of the unique timing of points, causing a lumpy or massive condition. Oil nitrogen fertilizer applications, much of the nitro- is added to reduce dust. A desirable granular fer- gen is applied alone. The mixing together of dif- tilizer contains hard granules of uniform size and ferent nutrient-containing materials produces composition and has good storage and handling mixed fertilizers that contain two or more plant qualities. Such materials are required by today's nutrients. Mixed fertilizer is commonly applied in modern fertilizer applicators. Granular fertilizers relatively small amounts with or near seeds for the are sold in bags or as bulk materials for direct purpose of accelerating early growth of crop spreading onto fields, and are also used to make plants. bulk blended fertilizers.

TABLE 14.3 Some Common Micronutrient Carriers MIXED FERTILIZERS 229

Bulk Blended Fertilizers plant foliage. They are either solutions or suspen- Granular fertilizers can be mixed together without sions. additional processing to produce bulk blends. The principal materials used for making solu- Such fertilizers, when handled in bulk form, are tions include urea, urea-ammonium nitrate solu- considerably cheaper than regular granular mixed tion, ammonium polyphosphate solution (10-34- fertilizers. Other advantages include the ease with 0), and finely ground KCI. Liquid fertilizer grades which a wide variety of grades can be produced are usually lower than dry fertilizers because of quickly and with negligible need for storage of the limited solubilities of the materials in water. In finished products. In many bulk-mixing plants, general, they are more expensive than dry fertil- the formula is calculated by using a computer izers. Additionally, low temperature can cause program, and the bulk blend is loaded into a truck crystallization, or salting-out of the salts. Most and immediately applied to fields. Currently, 54 popular liquid fertilizers have a salting-out tem- percent of the mixed fertilizers in the United States perature of freezing or below. A salting-out prob- are bulk blends. lem occurs only if liquids are stored in cold cli- Each of the materials in a bulk blend can sepa- mates during the winter months. The application rate from each other, or segregate, because no of liquid fertilizer is shown in Figure 14.7. chemical reaction between the materials has oc- Suspension fertilizers are manufactured from curred. Thus, a major precaution in bulk bending liquid fertilizers similar to those that are used to is that materials be of similar particle size; other- make solution fertilizers. Suspensions, however, wise, segregation will occur during handling and have more material added to the water than can application. The result is nonuniform distribution dissolve, so it becomes a thin slurry. One to 2 of the fertilizer nutrients in the field. In addition, percent clay (attapulgite, an alumino-Mg-silicate urea should never be added to a blend containing clay) is added to help hold particles in suspen- ammonium nitrate, because the blend adsorbs sion. This allows for preparation of higher grades, water from the atmosphere and cakes. as compared with liquid fertilizers. Conversely, salt crystals in suspension may grow over time and particles may settle out. This limits the length Fluid Fertilizers of the storage period. Fluid fertilizers are easily made and handled. Mi- The same amount of nutrients in fluid or dry cronutrients and pesticides are readily incorpo- form generally produces similar results when time rated. Fluid fertilizers can be applied to irrigation of application and soil placement are similar. Al- water and can be used for direct application to though fluids are very easy to handle, they need

FIGURE 14.7 Applying liquid fertilizer broadcast. (Photograph courtesy Ag- Chem Equipment Company.) 230 FERTILIZERS specialized application equipment, especially for terials with a C:N ratio of 20 or less are also valu- suspensions. The solution fertilizers are quite able as nitrogen sources. If material such as popular and often are surface applied with herbi- sawdust is used, it can be valuable as a mulch, cides mixed into the solution. Suspension fertil- but additional nitrogen is needed to prevent com- izers have attained only modest acceptance. petition for nitrogen between plants and microbes. Materials high in protein are high in nitrogen. Some nitrogen materials are listed in Table 14.4. NATURAL FERTILIZER MATERIALS Ground rock phosphate and waste pond or col- The emphasis in this section is on those generally loidal phosphate are sources of phosphorus. The available materials that have had minimal phosphorus is in calcium phosphate compounds, processing and that are sources of nutrients. They which are very slowly soluble in alkaline soils; the are of both organic and inorganic origin. same is true for bonemeal. These compounds are Animal manures have been used almost from more satisfactory when used on acid soils. the beginning of agriculture and are a good Animal tankage (refuse from animal meat pro- source of plant nutrients. A ton of wet (fresh) cessing) is also a source of phosphorus. The ap- animal manure contains approximately 10 proximate amount of phosphorus in some materi- pounds of nitrogen, phosphorus equivalent to 5 als is given in Table 14.4. pounds of P2 05 , and potassium equivalent to 10 Potassium is not an integral part of plant tis- pounds of K2 0. Animal manures are an excellent sues, and sources of potassium are inorganic or source of nutrients, because of the availability of from the ashes resulting from the burning of or- nutrients in the liquid portion and the decom- ganic materials. The potassium in ashes is in the posability of the solid fraction. Other organic ma- form of K2 CO3 . Repeated use of wood ashes may tend to cause dispersion of soil structure; the alkalinity effect may produce an undesirable increase in soil pH, because many plants grown on TABLE 14.4 Nutrient Composition of Some Natural Fertilizer Materials alkaline soil are prone to iron deficiency. Finely-ground granite is available, however; it contains highly insoluble potassium minerals such as feldspar and mica. The potassium in greensand (glauconite) is interlayer potassium in a micaeous mineral and similar to the potassium in the mica of granite fines. These sources of potassium have low availability. The composition of some potassium materials is given in Table 14.4. SUMMARY Fertilizers contain mainly soluble forms of plant nutrients. The grade indicates the nutrient composition. A grade of 27-3-9 contains on a weight basis 27 percent nitrogen, phosphorus equivalent Complied from several sources, including Taber, Davison, and to 3 percent P 2 0 5 , and potassium equivalent to 9 Telford, 1976. percent K2 0. REFERENCES 23 1

Most of the nitrogen in fertilizers comes from ganic materials are used as sources of nitrogen, fixation of atmospheric nitrogen. Reacting nitro- phosphorus, and potassium. The materials have gen with hydrogen produces ammonia, NH 3 . The lower nutrient content and generally fair to low reaction of ammonia with other chemicals pro- availability. duces a wide array of nitrogen fertilizers. The phosphorus in fertilizers comes from phosphate rock. The rock is acidulated with sulfuric or phosphoric acids to produce superphosphates REFERENCES that contain citrate soluble phosphorus. Phos- phoric acid is also used in the manufacture of Engelstad, O. P., ed. 1985. Fertilizer Technology and ammonium polyphosphates. Use, 3rd ed. Soil Sci. Soc. Am., Madison, Wis. The potassium in fertilizers comes mainly from Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. Wiley, evaporite deposits. The potassium chloride in the New York. International Fertilizer Development Center. 1979. deposits is recovered and marketed as 0-0-60. Fer- tilizer Manual. TVA. Muscle Shoals, Ala. Many micronutrient carriers are available to Taber, H. G., A. D. Davison, and H. S. Telford. 1976. supply micronutrients. "Organic Gardening." Washington State University Mixed fertilizers are marketed mainly as granu- Ext. EB 648. Pullman, Wash. lar, bulk blended, or fluid (liquid and suspension) Welch. L. F. 1986. "Nitrogen: Ag's Old Standby." Solu- fertilizers. tions: J. Fluid Fert. and Ag. Chem. July/August, pp. Various naturally occurring organic and inor- 18-21. CHAPTER 15

SOIL FERTILITY EVALUATION AND FERTILIZER USE

Soil fertility is the status of a soil with respect to its tion, however, is useful in determining which nu- ability to supply elements essential for plant trients are deficient. Lawns commonly turn light- growth without a toxic concentration of any ele- green or yellow when nitrogen is deficient, and ment. Soil fertility is determined or evaluated with this indication of nitrogen deficiency can be very various techniques, and this information is the useful for home owners. Many trees, shrubs, and basis for making fertilizer recommendations. The flowers develop iron deficiency symptoms when emphasis in this chapter is the evaluation of soil growing on alkaline soils and these symptoms are fertility, making fertilizer recommendations, and very important to many gardeners. Deficiency the use of fertilizers. symptoms can be useful to researchers in determining the location of soil fertility experiments on a deficient nutrient. Care must be exercised in using deficiency symptoms to evaluate soil fertil- SOIL FERTILITY EVALUATION ity, because several different deficiencies can pro- Evaluations of soil fertility are based on visual duce similar symptoms. observations and tests of both plants and soils. The major techniques used to evaluate soil fertility are plant deficiency symptoms, analysis of the Plant Tissue Tests nutrient content of plant tissue, and soil tests for Tissue or plant analyses are frequently used as the the amount of available nutrients. basis of fertilizer recommendations for crops that require one or more years to mature. This includes many tree crops, sugarcane, and pineap- Plant Deficiency Symptoms ple. The nutritional status of the plant can be Color Plates I to 4 in Chapter 12 contain defi- assessed and the appropriate fertilizer can be ap- ciency symptoms for several nutrients on several plied long before harvest, which allows plenty of different plants. By the time plant nutrient defi- time for the plants to benefit from the fertilization. ciency symptoms become obvious, it may be too As a result, plant analyses are popular with horti- late in the season to apply fertilizer. Such informa- culturists and foresters who grow tree crops. In

232 SOIL FERTILITY EVALUATION 233

addition, it is difficult to evaluate soil fertility for tree crops by using soil samples, because of the widespread and deeply penetrating root systems of trees and the difficulty of getting a representative soil sample. The use of diphenylamine as an indicator of the concentration of nitrate in the sap of corn is shown in Color Plate 1. Although the test is quali- tative, it is very useful. Other similar easy-to-use tests are available to verify plant nutrient deficiency symptoms. Simplicity of use and immediate results are beneficial to consultants and others who diagnose plant growth problems in the field. Exacting quantitative chemical tests of plant tissue are used to determine the degree of defi- FIGURE 15.1 Relationship between the potassium content of first-year needles of Red Pine, Pinus ciency or sufficiency of nutrients in plants. An resinosa, and site index or tree growth. (Data of enormous amount of research data has been sum- Stone et al., 1958.) marized into tables and figures that relate nutrient composition of particular plants to growth or yield. For example, corn plants have sufficient Plant analyses have been used to determine nitrogen for maximum yield when the nitrogen locations where the soil has sufficient available content of the leaf opposite and below the upper- nutrients to produce maximum yields in orchards most ear is 3 percent. without the use of additional fertilizer. For long- The relationship between the percent of po- maturing crops, such as sugarcane and pineap- tassium in first-year needles of Red Pine and ples, periodic plant analyses are used to produce growth, or plant height, is shown in Figure 15.1. a log of the nutritional status of the crop, and this Note that needles with less than 0.35 percent po- serves as a guide for proper timing of fertilizer tassium had deficiency symptoms. Growth was applications. maximum, with about 0.5 percent or more potassium in needles. To obtain the information needed to make a specific potassium fertilizer Soil Tests recommendation based on tissue analysis, the One of the weakest steps in making good fertilizer percentage of potassium in needles must be re- recommendations by using soil tests is the obtain- lated to the plant's response to potassium fertil- ing of a representative soil sample. A soil sample izer. For example, suppose in an experiment in a of 1 pint (about 1 pound or 500 grams) when used Red Pine plantation where the potassium content to represent 10 to 15 acres, means that 1 pint of of the needles was 0.25 percent potassium, that 50 soil is used to characterize 10,000 to 30,000 tons pounds of K2 0 per acre resulted in growth repre- of soil. sented by 60 percent of the relative maximum Fairly uniform areas of fields should be yield, and the application of 100 pounds of K 2 0 sampled separately on the basis of uniformity in produced growth equal to 100 percent of relative soil characteristics and past soil management. yield (maximum yield). A forester having a partic- Sampling of unusual areas, including the location ular goal for growth, could then fertilize accord- of manure or lime piles, should be avoided. Col- ingly, based on the experimental data. lect about 20 individual samples of the plow layer 234 SOIL FERTILITY EVALUATION AND FERTILIZER USE and mix them together in a clean plastic pail (see Figure 15.2). From this large soil sample, about 1 pint is sent to the testing lab. Because different labs use different kinds of tests and procedures, persons interested in soil tests should consult a soil testing lab for proper sampling procedures and care of samples after they have been collected. Most states have soil testing laboratories associated with both the Agricultural Experiment Station and agribusiness firms. Many fertilizer dis- tributors provide soil testing services for their cus- tomers. Shallower sampling is advised in no-till systems where nutrients accumulate in the upper few FIGURE 15.3 Distribution of soil phosphorus after inches of soil as a result of the surface application three years of tillage and fertilization. (From Potash of fertilizer (see Figure 15.3). In some instances, and Phosphate Institute, 1988.) samples of the subsoil are taken. Soil tests have been developed that measure relative yield of nearly 100 percent. This means the fraction of nutrients (available fraction) in the that a soil test of 61 or more pp2m is high and plow layer, which is closely related to plant indicates a situation where the application of growth. Soil tests for potassium, calcium, and phosphorus fertilizer is not expected to increase magnesium measure the exchangeable amounts. yield. The various soil test levels are categorized As with the procedures for developing fertilizer as high, medium, and low. For example, low recommendations based on plant tissue tests, soil phosphorus tests range from 16 to 30 pounds tests must be related to plant growth or crop yield. of phosphorus pp2m, and medium from 30 to For example, the relationship between phos- 61 pp2m. These results are based on the use of a phorus soil tests and the relative yield of corn in soil extractant composed of dilute HCI and NH 4 F, north-central Iowa is shown in Figure 15.4. Note called Bray's P1. This test is used in almost all that a soil test of 61 or more pp2m (pounds of labs in the midwestern region in the United States. phosphorus per acre-furrow-slice) resulted in a FIGURE 15.4 Relationship between phosphorus soil test and the growth of corn in north-central Iowa. FIGURE 15.2 Guides for taking soil samples. Avoid (Data courtesy of R. Voss, Iowa State University.) unsual areas and take about 20 random samples per area. SOIL FERTILITY EVALUATION 235

Some regions use other extractants because the mendations for potassium and some of the mi- soils contain different forms of phosphorus, and cronutrients are the same as those for phos- the results from Bray's P1 test do not relate well to phorus. Nitrogen fertilizer recommendations are crop yields. usually based on consideration of the nitrogen The next question involving the use of soil tests mineralizing power of the soil, the kind of pre- to make fertilizer recommendations is: If the soil vious and current crop, the application of ma- test for phosphorus is low, how much phos- nure, and the return of crop residues to the soil. phorus fertilizer will be required to obtain a partic- An example of soil test results and fertilizer ular crop-yield goal? In general, the most profit- recommendations is given in Figure 15.5. The soil able rate of fertilizer application is the amount testing laboratory report in this figure contains needed to obtain about 90 to 95 percent of the three sections: First, there is an identification sec- relative maximum yield. Data from experiments tion, giving sample numbers and record of the relating crop response to varying amounts of fer- previous crop. The soil analysis section gives the tilizer for various soil test levels are placed in soil test results for soil pH (and lime index), phos- tables, which are used to make fertilizer recom- phorus, potassium, calcium, magnesium, cation mendations. The phosphorus fertilizer recom- exchange capacity, and percentage of organic mendations for corn in north-central Iowa are matter. The fertilizer recommendations section given in Table 15.1. The data in this table also take contains the yield goals and recommendations for into consideration the level of subsoil phos- two or three years for various crops. phorus. A soil phosphorus test of 35 pp2m is Fertilizer recommendations for home gardens medium, and the phosphorus recommendation and lawns are much like those made for commer- for corn would be 70 pounds of P 2 0 5 per acre, if cial producers. For home gardens and lawns, subsoil phosphorus is very low or low, and 60 however, there is usually little, if any, profit motive pounds of P205 per acre if the subsoil test is involved, so a less rigorous procedure will satisfy medium or high in phosphorus. A series of tables the needs of many persons. For example, in the are constructed to make recommendations for absence of a soil test, a reasonable fertilizer appli- various crops in different climate and soil regions. cation for a lawn or garden is approximately 15 to The procedures for developing fertilizer recom- 20 pounds of 10-10-10 per 1,000 square feet. This is equivalent to 1.5 to 2 pounds of nitrogen, P205,

and K20 per 1,000 square feet. Many lawns and TABLE 15.1 Phosphorus Fertilizer gardens have been heavily fertilized for many Recommendations for Corn or Sorghum Grain years. Frequently, soil tests are high for certain in Iowa nutrients, which indicates that further fertilization Basic Recommended Amounts for Soil Test Class and is not needed. In these instances, the soil test Subsoil P Levels (P20 5 , lb/acre) indicates which nutrients are sufficient.

Computerized Fertilizer Recommendations The data in fertilizer recommendation tables are put into equations and fertilizer recommendations are generated by using a computer. Informa- tion regarding crop, soil, yield goals, soil pH, and other critical factors are entered, and the computer generates the recommendations and pro- 236 SOIL FERTILITY EVALUATION AND FERTILIZER USE

FIGURE 15.5 Sample of a computer-generated laboratory report showing soil test results and fertilizer recommendations. (Courtesy Dr. J. Alhrichs, Purdue University.) APPLICATION AND USE OF FERTILIZERS 237 vides a printout of soil test results and the recom- and yield expectations. For example, the nitrogen mendations (see Figure 15.5). Some specific recommendation for corn grain in Michigan is illustrations of the equations that are used for calculated with the following equation: computing fertilizer recommendations follow. The K 0 recommendation for potatoes grown pounds of fertilizer nitrogen per acre = 2 [ in Michigan is based on the following equation: -27+(1.36xYG)]-[40+(0.6xPS)]-(4xTM) Ks = 175 + (0.5 x YG) - (1.0 x ST) In this equation, YG is the yield goal in bushels per acre and PS refers to percent stand for le- where Ks is pounds of K2 0 recommended per gumes, as alfalfa and clover, that precede the corn acre for loamy sand and sandy loam soils, YG is crop. Five or six alfalfa plants per square foot yield goal in hundred weights per acre, and ST is is a 100 percent stand. A nitrogen credit of the soil test value in pounds of K pp2m(pounds 100 pounds is made when an alfalfa crop with per acre-furrow-slice). This recommendation 100 percent stand precedes the corn crop considers: (1) differences in the soil's ability to [40 + (0.6 x 100) = 100 pound credit]. The TM supply potassium by relating the recommen- refers to tons of manure, crediting each ton with 4 dation to soil texture, (2) yield goal sought by pounds of nitrogen fertilizer equivalent. Thus, the grower, and (3) the soil test level of the field. corn with an expected yield of 150 bushels per For a yield goal of 500 cwt/acre and a soil test acre and following an 80 percent stand of alfalfa of 200 pounds of potassium pp2m, the K 2 0 with 10 tons of manure applied per acre, will have recommended per acre is 225 pounds a nitrogen recommendation of 49 pounds per acre (175 + 250 - 200). (-27 + 204 - 88 - 40). For micronutrients, availability is closely re- Nitrogen does not accumulate and buildup in lated to soil pH so that soil tests for nutrients, plus soils where leaching occurs as do phosphorus soil pH, are used as the basis for recommenda- and potassium. For this reason, the nitrogen need tions. An example of a manganese recommenda- for a lawn remains nearly the same year after year. tion in pounds of manganese per acre for respon- Many home owners can maintain a good lawn sive crops is: fertilization program by applying only nitrogen pound of manganese per acre fertilizer. A moderate recommendation is 1 pound 136 + (6.2 x pH) - (0.35 x ST) of nitrogen per 1,000 square feet in the spring and again in late summer and/or fall. A serious gar- Here, the amount of manganese recommended is dener will be able to assess the nitrogen need of a directly related to soil pH and inversely related to lawn by careful observation of the color and the the manganese soil test (ST). In addition, differen- rate of growth when water does not seriously limit tial crop response to manganese is considered; growth. responsive crops include oats, beans, soybeans, and sugar beets. No commonly accepted soil test exists for making nitrogen fertilizer recommendations. Rather, a APPLICATION AND USE OF FERTILIZERS nitrogen balance system is commonly used, in After a fertilizer recommendation has been made, which estimates are made of the amount of nitro- the major factors to consider in the use of fertilizer gen needed by the crop and the amount of nitro- are time of application and placement. High and gen likely to be available. The amount of nitrogen very high soil tests mean that fertilizer is not ex- needed minus the amount of nitrogen available pected to increase plant growth or yields signifi- equals the nitrogen fertilizer recommendation. cantly. Obviously, under such conditions, a con- The recommendations vary greatly between crops sideration of time of application and placement of 238 SOIL FERTILITY EVALUATION AND FERTILIZER USE fertilizer is meaningless. In other words, the the soil along the rows when the corn is a foot or higher the soil fertility level, the less important are two high, and as a preplanting broadcast applica- time of application and placement. In many farm tion onto the soil surface in early spring were fields, however, time of application and place- much more effective than application the pre- ment are important considerations. vious fall. Side dress application resulted in providing nitrogen near the time of maximum plant need and with the least amount of time available Time of Application for nitrogen loss. The closer the time of fertilizer application is to The time of application of phosphorus and po- the time of maximum plant need, the more effi- tassium fertilizer is less critical than for nitrogen. cient plants will be in using the fertilizer nutrients. However, fixation of phosphorus, and to a lesser This is especially true for nitrogen, which is lost extent potassium fixation, reduce their availability by denitrification whenever the soil is anaerobic. and reduce the plant's ability to recover the ap- Nitrate is mobile and lost by leaching during pe- plied fertilizer phosphorus and potassium. The riods of surplus water, and NH3 is subject to vola- longer the period of time between application and tilization when it exists on the soil surface. This is plant use, the greater is the fixation and ineffi- shown in Figure 15.6, where nitrogen is applied as ciency of the nutrient use. In general, the levels of a side dress, placement of the nitrogen fertilizer in phosphorus and potassium in many soils in United States have increased to the point where time of application has little effect, and much of the phosphorus and potassium fertilizer is bulk- FIGURE 15.6 Effect of fall, spring, and sidedress nitrogen rates on corn yields at DeKalb, Illinois. blended and applied broadcast (spread uniformly Average for 1966-1969. (Reproduced from American on the soil surface) at a time when it is most Society of Agronomy, Vol. 63, No. 1, p. 119-123, convenient. copyright © 1971.)

Methods of Fertilier Placement Side dressing of nitrogen fertilizer for corn has already been mentioned as being an application of fertilizer along the rows after the corn is a foot or two high. A starter fertilizer application is one that contains a relatively small amount of nutrient elements and is applied with or near the seed in a band for the purpose of accelerating early growth of crop plants. Starter fertilizer is commonly applied an inch or two below and to the side of seeds, so that the nutrients will be near young roots and in moist soil. The result can be a faster start for the plants and a greater ability to compete with weeds. The earlier start, however, does not necessarily mean higher yields at harvest time. Another benefit of band placement is less contact of the soil with the phosphorus, thus reducing phosphorus fixation. The amount of fertilizer that can be safely APPLICATION AND USE OF FERTILIZERS 239 placed in a band near seeds is related to the salt volume versus the concentration of phosphorus effect of the fertilizer. As shown in Figure 15.7, and potassium in the soil solution. Consider that a fertilizer in contact with seed may reduce or given amount of phosphorus fertilizer is placed in prevent germination. If, however, the seed and a small volume of soil, and it produces a very high fertilizer are separated by a distance of only 2 to 3 concentration of phosphate ions in solution. centimeters (an inch), germination is barely af- Phosphorus uptake could be rapid, even with a fected. small amount of root length or surface area-den- Placement of nitrogen fertilizers is of little con- sity for absorption. This situation is representative cern because nitrate has great solubility and mo- of band placement. As the soil volume increases, bility. Essentially, all of the nitrate is in solution with a constant amount of fertilizer, more soil and and moves readily to roots by mass flow. The phosphorus contact results in greater phosphorus uptake of fertilizer phosphorus and potassium, adsorption (fixation), reducing the phosphorus compared with nitrogen, is more affected by low concentration in the soil solution. Conversely, a concentrations of phosphorus and potassium in greater soil volume means more roots and more the soil solution, and their uptake is more depen- root surface for phosphorus absorption. Theoreti- dent on rate of diffusion. The diffusion rates of cally, there is a most desirable volume for each phosphorus and potassium are so low that they soil into which the fertilizer should be mixed or move only 0.02 and 0.5 centimeters or less, replaced to achieve maximum uptake of phos- spectively, to roots in loam soils during the entire phorus, and potassium, and crop yield. Experi. growing season. mental data in Figure 15.8 show that when 50

The major effect of the phosphorus and po- pounds of P205 per 2 million pounds of soil were tassium fertilizer placement results from the net placed with varying amounts of the soil volume, effect of the amount of roots in the fertilized soil the maximum predicted phosphorus uptake occurred with soil volumes ranging from 2 to 20 percent of the soil volume, depending on soil. The greater the amount of available phosphorus al- FIGURE 15.7 Effects of fertilizer placement (shown ready in the soil, the larger the volume of fertilized by arrows) on the germination of wheat. On left it is with seed and, on the right, the fertilizer and seeds soil for maximum phosphorus uptake. Localized are separated. placement of phosphorus in a band along the crop rows is especially effective in intensively weathered soils. These soils are rich in iron and aluminum oxides, have high phosphorus fixation capacity and, generally, have a very low phosphate concentration in the soil solution. In summary, localized placement of fertilizer in a band puts fertilizer in contact with a small soil volume in which there are few roots; however, there is a high concentration of fertilizer nutrients in solution. By comparison, when fertilizer is broadcast uniformly over the entire soil surface, the fertilizer is eventually mixed with a very large soil volume. This results in many roots in the soil volume but a much lower concentration of fertilizer nutrients in solution. 240 SOIL FERTILITY EVALUATION AND FERTILIZER USE

cal method used to fertilize a lawn, forage field, or forest. The fertilizer is placed on the soil surface. It may be ineffectively used if the soil surface remains dry. Rain and irrigation water move nitrogen rapidly into the soil, but they move potassium to a lesser extent, and phosphorus very little. In general, the mobility of nitrogen as nitrate in soils results in little if any placement effect for nitrate fertilizers. Urea, and other NH3-producing fertilizers, including animal manures, are subject to significant nitrogen loss by volatilization when broadcast onto the soil surface, and the losses cannot be easily avoided. Fertilizer application by airplane or helicopter is common for flooded rice fields, forests, and rugged pasturelands. Fertilizer is added to irrigation water and is applied to leaves as a foliar spray. Foliar sprays are important for supplying plants with certain micronutrients, such as iron, FIGURE 15.8 The effect of fraction of soil fertilized on predicted uptake of phosphorus by corn plants that are rapidly precipitated when a soluble form is added to the soil as fertilizer. from the application of 50 pounds of P205 per acre. Uptake values are relative. The Raub soil has the Capsules can be inserted into tree trunks to highest soil test for phosphorus, and the Pope soil supply certain micronutrients. Fertilizer "spikes" has the lowest. (Data from Kovar and Barber, 1987.) can be inserted in the soil of container grown plants to increase the supply of nutrients in the Broadcast fertilizer application is the surface soil solution. application in a more or less uniform rate (see Figure 15.9). If the broadcast application is made Salinity and Acidity Effects after the crop has been planted, it is called top Fertilizers are soluble salts and their effect is to dressing. Broadcasting or top dressing is the typi- lower the water potential of the soil. The rate of

FIGURE 15.9 Broadcast application of dry fertilizer. (Photograph courtesy Ag- Chem Company.) ANIMAL MANURES 241

water uptake by seeds and roots is reduced. Fertil- Ammonium chloride is not a common fertilizer; izers placed directly with seeds can delay and/or however, in an experiment it produced similar reduce germination, as shown in Figure 15.7. Bio- results on soil pH and crop yields as did ammo- logical oxidation of ammonia or ammonium, NH 3 nium sulfate. or NH4', produces H+ and contributes to soil acidity. Continuous corn grown on acid soils that is fertilized mainly with NH3 can eventually result in very acid soils and accompanying acid soil ANIMAL MANURES infertility. Toxicities of manganese and/or alumi- After centuries during which animal manure was num and a deficiency of molybdenum are likely. highly prized for its fertilizing value, manure on Each pound of nitrogen from ammonium sul- many American farms has become a liability. Spe- fate produces acidity equal to that neutralized by cialized animal farms with many animals produce about 5.5 pounds of CaC0 3 . Ammonium nitrate, more manure than can be used effectively on the anhydrous ammonia (NH 3), and urea produce cropland. The emphasis has changed from the acidity equivalent to about 1.8 pounds of CaC0 3 application of manure onto soils to increase soil for each pound of nitrogen. Where increased soil fertility to the use of the soil for animal waste acidity is desirable, (NH4 ) 2 SO 4 is recommended disposal, as shown in Figure 15.11. The increas- as the nitrogen fertilizer; it is frequently used for ing shortage of fuel wood has increased the use of blueberry production. The acidity effects of phos- manure as a source of energy in developing phorus and potassium fertilizers are generally countries. unimportant, except for that of the ammonium nitrogen in ammonium phosphates. The wheat shown in Figure 15.10 failed to grow because of Manure Composition and acidity produced by heavy and continued applica- Nutrient Value tion of ammonmiurn chloride over several years. Many factors affect the quantity of manure produced and its composition, including: (1) the kind and age of the animal, (2) the kind and FIGURE 15.10 Very little wheat was produced on amount of feed consumed, (3) the condition of the plot in the foreground with a pH of 3.7, because the animal, and (4) the milk produced or work of long and continued application of ammonium performed by the animal. chloride. Note excellent wheat growth on the plot in Farm manures consist of solids and liquids. the rear with a favorable soil pH. The phosphorus and dry matter are concentrated in the solid portion (feces). On average, the liquid portion of cattle manure contains about two thirds of the potassium and one half of the nitrogen as shown in Figure 15.12. Because the composition of manure is so variable, data on manure composition presented in Table 15.2 can only be approximate. Earlier, it was shown that a ton of average farm manure was considered the equivalent of 4 pounds of nitrogen in fertilizer for corn grain production. The phosphorus and potassium in a ton of manure are equated to about 2 pounds of P 2 0 5 and 8 pounds of K20 from fertilizer. The nutrient value of the manure from 500 beef cattle is 242 SOIL FERTILITY EVALUATION AND FERTILIZER USE

FIGURE 15.11 Aerial view of large feedlot at Coalinga, California. The waste-disposal problem for each 10,000 cattle is equal to that of a city of 164,000. (Photograph courtesy Environmental Protection Agency.)

$19,600 per year. This is based on the assump- ally due to NH3 . Manure contains many microbial tions that the cattle weigh 750 pounds, and the decomposers, and there is a continuous forma- value of each pound of nutrients is worth 18 cents tion and loss of NH 3 as long as favorable condi- for nitrogen, 21 cents for P 2 0 5 , and 9 cents tions exist. After manure is spread in the field, for K20. drying and/or freezing can accentuate NH 3 loss. As a result, only a few practical things can be done to conserve the nitrogen in manure during Nitrogen Volatilization Loss storage and application. During storage, one or a from Manure few large piles is better than many small piles of Large amounts of NH 3 are produced in manure, manure. The sooner the manure is incorporated especially by the decomposition of urea and uric into the soil after spreading, the more nitrogen is acids. The pungent odor in a closed barn is usu- conserved. An ideal handling of manure occurs

FIGURE 15.12 Distribution of nitrogen, phosphorus, potassium, and dry matter in cattle manure. (From Azevedo and Stout, 1974.) ANIMAL MANURES 243

Compiled from Van Slyke, 1932. ° Multiply by 0.5 to convert to kilograms per metric ton. Clear manure without bedding; tons excreted by 1,000 lb of live weight of various animals.

when manure in pen or stall barns is made engines. The sludge and effluent are excellent for anaerobic by cattle tramping, the manure is occa- use as fertilizer. A biogas generator, with a metal sionally removed from these barns and then floating digestion chamber, is shown in Figure plowed under immediately after application. 15.13. The dairy farm with the largest herd of Many farmers are moving to a liquid manure- registered Holstein cattle in the United States has handling system. The manure is accumulated in a biogas generation plant that produces all the pits and the liquid manure is injected into the soil. electricity needed to operate the farm. Several million biogas generators are used in China, where the primary interest is in the nutrient Manure as a Source of Energ value of the digested sludge and the effluent. Be- Biogas production using manure and other or- cause most harmful disease organisms are killed ganic wastes is based on anaerobic decom- in digestion, biogas generation is also a good position of the wastes. During anaerobic decom- means for disposing of human wastes and for position, the major gas produced is methane keeping the environment more sanitary. The gas (CH4). The remaining manure sludge and liquid is combustible and so safety precautions are effluent contain the original plant-essential ele- needed to prevent an explosion resulting from ments of the digested materials. Because much exposure of the gas to an open flame. carbon is lost as C0 2 , the sludge and effluent are During the settlement of the Great Plains in the enriched with nitrogen and phosphorus, relative United states, bison manure (chips) was used to the original materials. for fuel. Today, many people live in areas that Basically, a slurry of manure and other organic are not forested and fuel wood is scarce. In wastes is fed into a hopper that is connected to a India, about 80 percent of the cattle manure is microbial digestion chamber. The gas produced formed in cakes, dried, and used as fuel (see is piped off to light lamps, cook food, or run small Figure 15.14). 244 SOIL FERTILITY EVALUATION AND FERTILIZER USE

FIGURE 15.13 Biogas generator with intake hopper (at left), floating metal gas holder and anaerobic digestion chamber in the center, and sludge discharge at the right.

beginning of an era when land disposal of sewage LAND APPLICATION OF wastes is being intensively studied and accepted SEWAGE SLUDGE by the public. Sewage sludge is the solid residue Land application of wastes to soils has been prac- remaining after sewage treatment. The increasing ticed since ancient times and still represents an cost of disposal by sanitary landfills has increased important part of many soil management pro- interest in land disposal. Seventy to 80 percent of grams. The federal mandate on water pollution the municipal sewage treatment plants in Michi- control and waste disposal, however, marks the gan dispose of sludge by land application. About 2 percent of the land in the United States would be required to dispose of all of the sludge produced. FIGURE 15.14 Cow dung shaped into cakes, plastered on a wall, and allowed to dry. The dried chips are used for cooking food where fuel wood is in short supply. Sludge as a Nutrient Source Sewage sludge is used as a source of nutrients for crop production. The median contents, on a dry- weight basis, of sewage sludge produced in the United States is about 3 percent nitrogen, 2 percent phosphorus, and 0.3 percent potassium. A 10-ton application per acre would contain 600 pounds of nitrogen, which is mainly organic nitrogen, and about 20 to 25 percent is estimated to be mineralized the first year. A 10-ton application would supply about 120 to 150 pounds of nitrogen for plant uptake. The regular nitrogen recommendation for crops could then be reduced by this LAND APPLICATION OF SEWAGE SLUDGE 245 amount. About 3 percent of the nitrogen remain- sues. Potential hazards of heavy metals in the ing in the sludge will be mineralized in the sec- environment are discussed in Chapter 13. ond, third, and fourth years after application. The heavy metal content of the sludge limits the Where sludge is applied repeatedly, allowance total amount of sludge that may be applied to must be made for the release of nitrogen from land. Metal loading values have been developed, residual-sludge. Excessive applications of sludge which are the total amounts of the heavy metals result in reduced yields, so there is a limit on how that can be applied before further sludge applica- much sludge can be applied to land and still tion is disallowed. The reaction and fixation of produce acceptable yields. Excessive application heavy metals into insoluble forms are related to of sludge also poses a hazard for pollution of soil texture. Therefore, loading values for heavy groundwater with nitrate. metal application on agricultural land have been The phosphorus in sludge is equated to that in tied to the cation exchange capacity, which is an fertilizers. The phosphorus content of sludge is indication of the organic matter and clay content typically high enough when using sludge to satisfy in soils. Loading values are given in Table 15.3. all or meet most of the nitrogen needs of crops, An average loading value for cadmium is 10 that more phosphorus will be applied in the pounds per acre. For a sludge containing 14 ppm sludge than is needed. This is not a serious prob- of cadmium, there is 0.028 pounds of cadmium lem, because soils have great capacity to fix phos- per ton. The amount of sludge that would need to phorus. On coarse-textured soils very high levels be applied to attain the cadmium loading value of of soil phosphorus could eventually pose a haz- 10 pounds per acre is 357 tons. ard for leaching of some phosphorus to the Heavy metals are less soluble in alkaline soils, groundwater. and maintaining high soil pH is one way to reduce The potassium content of the 10 tons of sludge the likelihood of heavy metal contamination of would be equated to about 60 pounds of fertilizer food crops. Soil pH should be maintained at 6.5 or potassium. higher. The lime requirement of soils receiving Solid dry-sludge application to the land as a sludge is that amount of lime needed to increase broadcast application may encounter an odor soil pH to 6.5. The great importance of soil pH in problem, which is why sludge is sometimes han- deactivating heavy metals has overshadowed the dled as a slurry and injected directly into the soil. This has the added advantage of greatly reducing nitrogen loss by volatilization. TABLE 15.3 Total Amount of Sludge Metals Allowed on Agricultural Land

Heav Metal Contamination Certain industrial processes, such as metal plat- ing, contribute heavy metals to sludges. Among these are cadimum (Cd), lead (Pb), zinc (Zn), nickel (Ni), and copper (Cu). The greatest detrimental impact of applying sludge to agricultural land is likely to be associated with the cadmium content of the sludge. Many crops may contain elevated concentrations of cadmium in their vegetative tissues without showing symptoms of toxicity. However, cadmium increases in the grain have been much lower than in the vegetative tis- 246 SOIL FERTILITY EVALUATION AND FERTILIZER USE importance of cation exchange capacity as the creased growth of algae and other aquatic plants, basis for determining heavy metal loading values. and their subsequent microbial decomposition, depletes the water of oxygen and kills fish living in the water. Sometimes, excess weed growth ham- pers swimming and boating. The real problem is FERTILIZER USE AND caused by erosion of soil rather than the use of ENVIRONMENTAL QUALITY phosphorus fertilizer. In fact, much of the pol- Environmental concerns regarding fertilizer, ma- lution occurs at construction sites where unpro- nure, and sludge use center around the pollution tected soil is exposed to serious erosion. Where of surface waters with phosphate and pollution of phosphorus fertilizer increases the vigor of a vege- groundwater with nitrate-N. tative cover, erosion and pollution of surface waters are reduced. In some states, one half or more of the soil Phosphate Pollution samples test high or very high for phosphorus. Phosphate from fertilizer and organic wastes re- More and more farmers are receiving fertilizer rec- acts with soil and becomes adsorbed (fixed) onto ommendations for only small maintenance levels soil particles. The adsorbed H2P04 - is carried of phosphorus, which should help reduce the along and deposited in surface waters as soil sedi- danger of phosphorus pollution of waters. ment because of erosion. This adsorbed phosphate in the sediment establishes an equilibrium concentration of phosphate with the surrounding Nitrate Pollution water, just as it does in the soil with the soil Groundwater naturally contains some nitrate - solution. The result is an increased level of avail- (N03 ). Since ancient times, the disposal of hu- able phosphorus for plants growing in the water man and animal wastes from villages created - because of the desorption of the H2 P0 4 . In- varying degrees of nitrate pollution. Today,

FIGURE 15.15 Relationship between the amount of applied nitrogen fertilizer, corn yield, and the amount of nitrogen recovered in grain and remaining in soil as leachable nitrogen. (Data of Broadbent and Rauschkolb, 1977.) SUSTAINABLE AGRICULTURE 247

however, the problem is more serious because of nitrate accumulates in the plant. Sometimes as the extensive and high rates of nitrogen fertilizer much as 5 percent or more of a plant's dry weight used for crop production, and the concentration may be nitrate-nitrogen. Under these conditions, a of animals on large farms and ranches. Some of high concentration of nitrite may be produced in the initial concerns of nitrate pollution in the the digestive tract of ruminants (cud-chewing United States were the result of the pollution of animals). Nitrites are very toxic and interfere with water wells on farms. These wells became pol- the transport of oxygen by the bloodstream. luted with nitrate produced in barnyards from High nitrate levels in plants usually result from manure. high soil nitrate plus some environmental factor The extent to which nitrate moves to the water that results in nitrate accumulation in plants. Dur- table as water passes through soil is a function of ing a drought, plants absorb nitrates, but the lack the amount of nitrate in the soil and the amount of of water interferes with the normal utilization of nitrate immobilized by the crop and microor- the nitrate for growth. Then, nitrate accumulates ganisms. Thus, the use of nitrogen fertilizer could in plants. Other factors that may contribute to have no effect or it could increase or decrease the nitrate accumulation are cloudy weather and re- amount of nitrate leaching through the soil to the duced photosynthesis. water table. Nitrate pollution (an increase in the Various plants show different tendencies to ac- nitrate level of groundwater above the normal cumulate nitrate. Annual grasses, cereals cut at level) is a problem when more nitrogen fertilizer the hay stage, some of the leafy vegetables, and is added than crops and microorganisms can im- some annual weeds are most likely to contain mobilize or absorb during growth. The data in high nitrate levels. Pigweed is a nitrate accumula- Figure 15.15 show that when more than 220 tor. High concentrations of nitrate are less likely pounds of nitrogen per acre were applied, the to accumulate in legumes and perennial grasses, crop yield and uptake of nitrogen did not continue but even these species are not completely free to increase significantly, and additional fertilizer from the problem. Within any one plant, the ni- nitrogen appeared as nitrate that could be trate level in the seeds or grain is almost always leached from the soil. lower than in the leaves. High levels of nitrate The nitrate pollution problem can be solved by have not been found in the grains used as food making a realistic assessment of yield goals and and feed crops. the application of nitrogen fertilizer and organic Monogastric (single-stomached) animals are wastes accordingly. Efforts are being made by not likely to develop a nitrite problem from the many Agricultural Experiment Station and Agri- ingestion of nitrates. No authenticated cases of cultural Extension personnel in various states to human poisoning have been traced to high levels encourage formers to avoid excessive application of nitrate in U.S. food crops. Cases of nitrite poi- rates of both nitrogen and phosphorus fertilizers. soning have been reported in European infants as Recommendations of nitrogen for soybeans are a result of consumption of high nitrate-content being reduced or eliminated. vegetable baby foods that were improperly stored.,

Nitrate Toxicity SUSTAINABLE AGRICULTURE Nitrate is absorbed by roots, translocated to the shoots, and converted into proteins. Usually, the Western agriculture, characterized by high tech- nitrate content of food and feed crops is not toxic nology, has been considered to be efficient. The to animals. Whenever nitrate moves into the plant quote "one farmer producing enough food to feed more rapidly than it is converted into protein, 70 people" fails to convey the fact that four 248 SOIL FERTILITY EVALUATION AND FERTILIZER USE tractors, thousands of dollars of capital invest- ever. Significant movement toward and accep- ment, large trucks, miles of concete highway, and tance of a restructing of agriculture to a sustain- people in processing and marketing activities able mode will require a change in society's choice are required. Now, there is growing concern indicators. That is, total costs of production must about future energy supplies and costs. Addi- include offsite costs and environmental damage, tionally, today's agriculture experts are finding and a desire for economic profit in the short term that the solution to one problem frequently will need to be tempered by a greater concern for creates other, more difficult problems. Many the quality of life over the long term. farmers are seeking alternative farming methods, because costs seem to be overtaking the benefits from the adoption of newer technology. SUMMARY Another aspect of the interest in sustainable agriculture is the concern over food contamina- Soil fertility is the soil's ability to provide essential tion from harmful chemicals. For many years, or- elements for plant growth without a toxic concen- ganic gardeners have been devising methods to tration of any element. Soil fertility is assessed by grow crops and animals without chemical fertil- use of deficiency symptoms, plant tissue analysis, izers, pesticides, and animal growth stimulators. and soil testing. For some, farming and food production have a Plant tissue analysis and soil tests are the basis "holistic" component that is reflected in a certain for making fertilizer recommendations. personal philosophy and way of life. Fertilizer nutrients are absorbed most effi- Slash-and-burn farmers in the humid tropics ciently when they are applied near the time of have a self-sustaining system that is ecologically maximum plant uptake, owing to the potential sound and permanent, provided the population loss of nitrogen by leaching and volatilization and does not overwhelm the system and make fallow the fixation of phosphorus and potassium. These periods too short to regenerate the soil. Many factors are also important considerations for subsistence farmers throughout the world have placement of fertilizers. minimal reliance on outside production inputs. In some cases, the acidity and salinity effects of Agricultural development in Europe and the fertilizers are important considerations in the se- United States was based largely on self-sustaining lection and application-of fertilizers. systems. During the nineteenth century, crop rota- Manure contains nutrients and is a substitute tions, including legumes to fix nitrogen, and for chemical fertilizers. Manure is also an energy animals to provide both power and cycling of source for biogas generation and as a substitute nutrients in manure, resulted in fairly satisfactory for fuel wood. control of pests and fairly high yields. Farm ma- Much of the sewage sludge produced in the nures were highly valued for their nutrient United States is applied to land. This sludge con- content. tains nutrients that can substitute for fertilizers. Crop production research in this area has pro- Some sludges contain heavy metals, and the duced promising results. In some cases, the re- amount of sludge that can be applied is limited to sults of reduced crop yields have been canceled the heavy metal content of the sludge. Soil pH out by lower input costs. This problem, however, should be maintained at 6.5 or higher to inactivate is not only a technological one and it is not an heavy metals. isolated one within society. Rather, it is an expres- Modern agriculture is energy intensive and has sion of societal concerns that invades all areas of been becoming increasingly so. Sustainable agri- life. Technology has brought apparent abundance culture concerns stem from the uncertainity of the to millions, yet millions are as improvished as future supply and cost of energy. REFERENCES 249

REFERENCES waters and Sludges." North. Cent. Reg. Ext. Pub. 52. Michigan State University, East Lansing. Anonymous. 1986. Utilization, Treatment, and Disposal Kovar, J. L. and S. A. Barber. 1987. "Placing Phosphorus of Waste on Land. Soil Science Soc. Am., Madison, and Potassium for Greatest Recovery." Jour. Fert. Is- Wis. sues. 4(1):1-6. Anonymous. 1988. Fertilizer Management for Today's Neely, D. 1973. "Pin Oak Chlorosis-Trunk Implantations Tillage Systems. Potash and Phosphate Institute, At- Correct Iron Deficiency." Jour. For. 71:340-342. lanta, Ga. Page, A. L., T. J. Logan, and J. A. Ryan. 1989. Land Azevedo, J. and P. R. Stout. 1974. "Farm Animal Ma- Application of Sludge. Lewis Publishers, Chelsea, nures: An Overview of Their Role in the Agricultural Mich. Environment." Ca. Agr. Exp. Sta. Manual 44, Davis, Stoeckeler, J. H. and H. F. Arneman. 1960. Fertilizers in Cal. Forestry. Adv. Agron. 12:127-165. Bezdicek, D. F. and J. F. Power (eds.) 1984. "Organic Stone, E. L., G. Taylor, and J. DeMent. 1958. "Soil and Farming: Current Technology and its Role in a Sus- Species Adaption: Red Pine Plantations in New tainable Agriculture." ASA Spec. Pub. No 46, Amer. York." First North Am. Forest Soils Conf. Proc. Michi- Soc. Agronomy, Madison, Wis. gan State University, East Lansing. Broadbent, F. E. and R. S. Rauschkolb. 1977. "Nitrogen Van Slyke, L. L. 1932. Fertilizers and Crop Production. Fertilization and Water Pollution." CaliforniaAgricul- Orange Judd, New York. ture, 31 (5):24-25. Voss, R. 1982. "General Guide for Fertilizer Recommen- Cressman, D. R. 1989. "The Promise of Low-input Agri- dations in Iowa. "Agronomy 65. Iowa State University, culture." J. Soil and Water Con. 44:98. Ames. Edens, T. C., C. Fridgen, and S. L. Battenfield. 1985. Warncke, D. D., D. R. Christensen, and M. L. Vitosh. Sustainable Agriculture and Integrated Farming 1985. "Fertilizer Recommendations: Vegetable and Systems. Michigan State University Press, East Field Crops in Michigan." Mich. State Univ. Ext. Bull. Lansing. E-550. East Lansing, Mich. Engelstad, O. P. (ed.) 1985. Fertilizer Technology and Welch, L. F., D. L. Mulvaney, M. G. Oldham, L. V. Use. 3rd ed. Soil Sci. Soc. Am., Madison, Wis. Boone, and J. W. Pendleton. 1971. "Corn Yields with FAO. 1978. China: Azolla Propagation and Small-scale Fall, Spring, and Sidedress Nitrogen." Agron. Jour. Biogas Technology. FAO Soils Bull, 41, Rome. 63:119-123. Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. Wiley, Welch. L. F. 1986. "Nitrogen: Ag's Old Standby." Solu- New York. tions: J. Fluid Fert. and Ag. Chem. July/August, pp. Illinois Agronomy Department. 1984. Illinois Agron- 18-21. omy Handbook 1985-1986. University of Illinois, Wolcott, A. R., H. D. Foth, J. F. Davis, and J. C. Urbana. Shickluna. 1965. "Nitrogen Carriers: Soil Effects." Jacobs, L. W. 1977. "Utilizing Municipal Sewage Waste- Soil Sci. Soc. Am. Proc. 29:405-410. CHAPTER 16

SOIL GENESIS

Soil genesis deals with the factors and processes the other soil-forming factors must remain con- of soil formation. The formation of soil is the stant, or nearly so. result of the interaction of five soil-forming factors: parent material, climate, organisms, topographic position or slope, and time. The domi- A Case Study of Soil Genesis nant processes in soil genesis are: (1) mineral At the end of the Wisconsin glaciation, about weathering, (2) humification of organic matter, 10,000 years ago in the upper Midwest of the (3) leaching and removal of soluble materials, United States, the Great Lakes were larger than and (4) translocation of colloids (mainly silicate they are today. The lowering of lake levels oc- clays, humus, and iron and aluminum oxides). curred in stages to produce a series of exposed The net effect of these processes is the develop- beach surfaces of different ages. In the case of the ment of soil horizons, that is, the genesis of soil. study area, there are three different soils on lake beaches aged 2,250 years, 3,000 years, and 8,000 years. All slopes were nearly level. Similar cal- ROLE OF TIME IN SOIL GENESIS careous, quartzitic sand parent material occurred at each location, as shown in Figure 16.1. Soils are products of evolution, and soil proper- For a short period of time after the glacial ice ties are a function of time or soil age. The age of a melted, a tundra climate existed. Soon after, the soil is expressed by its degree of development and climate became favorable for trees. The climate not the absolute number of years. In a sense, soils went through a warming trend between 4,000 and have a life cycle that is represented by various 6,000 years ago; today, the climate is humid- stages of development. Reference has been made temperate. In general, the vegetation was a mixed to minimally, moderately, and intensively weath- coniferous-deciduous forest. ered soils. To study time as a soil-forming factor, Two factors that greatly affected soil genesis in

250 ROLE OF TIME IN SOIL GENESIS 251

FIGURE 16.1 Relationship between lake beach locations and ages for soil genesis in northern Michigan. Ages of deposits given as years before the present (BP). (Data from Franzmeier and Whiteside, 1963.)

this study were (1) the quartzitic sand-parent ma- horizon (h from humus). This 8,000-year-old soil terial, and (2) a humid climate that produces has A, E, Bhs, and C horizons and is called the much leaching. Very little clay existed in the par- Kalkaska soil. A soil similar to the Kalkaska is ent material and very little clay was formed via shown in Figure 16.2 (also see book cover). weathering. Clay translocation was insignificant, Note from Table 16.1, that as colloidal and and Bt horizons could not develop. Under the amorphous sesquioxides and humus (noncrys- conditions of a high-leaching regime, colloidal talline materials with high specific surface) accu- humus and oxides of iron and aluminum (sesqui- mulated in the subsoil, the tree species changed. oxides) were translocated downward and accu- The change was toward species more demanding mulated in the subsoil. of water and nutrients, and this parallels the soil's The changes that occurred during soil genesis ability to retain more water and nutrients as are summarized in Table 16.1. During the first amorphous colloidal materials accumulated in 2,250 years, humification and accumulation of the B horizon. Therefore, as the soils evolved, soil organic matter near the soil surface resulted in the properties changed and this caused a change in formation of an A horizon. The lime present in the the composition of the forest species on each upper part of the parent material was leached out terrace. The most productive soil in the sequence and the A horizon became acid. The acid A hori- for forestry is the Kalkaska soil. zon overlies a slightly discolored parent material layer. This young soil, which has only A and C horizons, is the Deer Park soil. During the next 750 years, leaching continued Time and Soil Development Sequences to remove lime, and acidity developed in the soil It should be recognized that the rate of soil gene- to a depth of 1 meter or more. Sufficient sesquiox- sis in a particular situation is highly dependent on ides accumulated in the subsoil to form a Bs the nature of the soil-forming factors. In some horizon (s from sesquioxide). Simultaneously, an situations, soil genesis is rapid, whereas in oth- E horizon evolved. This 3,000-year-old soil on the ers, soil genesis is very slow. Nipissing beach has A, E, Bs, and C horizons and Some soil features or properties develop is called the Rubicon soil. quickly, but the development of other properties During the next 5,000 years, there was consider- requires much more time. able movement and accumulation of humus in A horizons can develop in a few decades. Con- the subsoil, and the Bs horizon became a Bhs sequently, soils with only A and C horizons can 252 SOIL GENESIS

TABLE 16.1 Major Processes and Changes During Soil Evolution in Northern Michigan From Sand Parent Material

Adapted from Franzmeier and Whiteside, 1963.

develop in 100 years or less. The formation of a num is approximately 100,000 years. Some of Bw horizon involves changes in color and/or these soils in the tropics are more than a million structure and requires about 100 to 1,000 years. years old. Soils with an A-Bw-C horizon sequence, therefore, require commonly several hundred years for development. The development of the Bt horizon is ROLE OF PARENT MATERIAL IN dependent on some clay formation within the soil SOIL GENESIS and significant translocation of clay. These processes usually require a minimum of several Parent material has a great influence on the thousand years. Soils with an A-E-Bt-C horizon properties of young soils, including color, texture, sequence need about 5,000 to 10,000 years to structure, mineralogy, and pH. Over time the ef- form under favorable conditions. The time to form fects of the parent material decrease, but some intensively weathered soils containing a high effects of the parent material still persist in old content of kaolinite and oxides of iron and alumi- soils. ROLE OF PARENT MATERIAL IN SOIL GENESIS 253

the early stages of rock weathering and soil formation, the formation of parent material and soil may occur simultaneously as two overlapping processes. Soils formed in parent materials derived from the weathering of underlying rocks are common in the U.S. Appalachian Highlands and Interior Highlands in the eastern states and in the Cordilleran Region in the western states (see Fig- ure 16.3). Within these regions, however, many soils have formed in the sediments that occur at the base of steep slopes or in small depressions, basins, or areas beside streams and rivers.

Soil Formation from Limestone Weathering Parent material formation from most hard rocks is by the physical disintegration and chemical decomposition of the mineral particles in the rock. A parent material derived from sandstone is, essentially, composed of the sand particles that were cemented together in the sandstone. In the case of limestone, the soil develops in the insoluble impurities that remain after the calcium and magnesium carbonate have been dissolved and leached from the weathering environment. Clay is a common impurity in limestone, giving rise to the fine texture of soils derived from limestone weather- FIGURE 16.2 Soil with A, E, Bhs, and C horizons. ing. Chert is a compact, siliceous material that (The metal frame is used to collect a vertical slab of occurs in some limestones. The chert resists dis- soil to mount on a board and treat with plastic, to be solution, and soils formed from the weathering of used for display purposes.) cherty limestone are stony. Many centimeters of limestone are required to form a centimeter of soil, because the impurities (residues) typically comprise only a small percentage of the limestone and some of the residues Consolidated Rock as a Source of are carried away. It has been estimated that a foot Parent Material of soil accumulation in the Blue Grass region of Consolidated rocks are not parent material but Kentucky required limestone of the order of 100 serve as a source of soil parent material. Soil feet and more than half a million years. Most soils formation may begin immediately after the depo- of the Kentucky Blue Grass region of Kentucky sition of volcanic ash but must await the physical and the Nashville Basin of Tennessee have devel- disintegration of hard rocks when granite, basalt, oped from limestone and were prized by settlers or sandstone are exposed to weathering. During for their suitability for agriculture. A soil devel- 254 SOIL GENESIS

FIGURE 16.3 Physiographic regions of North America.

oped from limestone weathering is shown in Fig- FIGURE 16.4 Parent material derived from ure 16-4. limestone is the residual accumulation of impurities in the limestone after water has leached away the carbonate materials. Because clay is a major impurity, parent materials and soils derived from Sediments as a Source of limestone weathering tend to be fine textured. Parent Material Most soils have developed from sediments that were transported by water, wind, ice, or gravity. Colluvial sediments occur at the base of steep slopes where gravity is the dominant force, causing movement and sedimentation. Colluvial sediments are common and are an important parent material in mountainous areas. Alluvial sediments are ubiquitious because of the widepread existence of streams and rivers (see Figure 16.5). The most extensive area of alluvial sediments in the United States occurs along the Mississippi River. Most of the agricultural soils ROLE OF PARENT MATERIAL IN SOIL GENESIS 255 in California occur in valleys where alluvium The majority of the other soils on the coastal is the dominant parent material. Colluvial and plains developed in sediments consisting mainly alluvial sediments occur as inclusions in areas of quartz sand and kaolinite clay. The quartz and dominated by hard rocks or other kinds of sedi- kaolinite represent the "end products" of highly ments. Regions of the United States in which weathered sediments and soils. The sediments sediments are extensive and important sources accumulated over a long period of time and some of parent material are shown in Figure 16.3. of the material in the sediments participated in The regions include the Gulf and Atlantic several weathering cycles before coming to rest in Coastal Plains, the Central Lowlands, the the sea. The coastal plains slope downward to- Interior Plains, and the Basin and Range. ward the sea, resulting in increasing age of sedi- Sediments were the most common source of ments and a period of soil formation from the parent material for the soils of the United seacoast inward. In the southeastern United States. States this has produced three major surfaces- the lower, middle, and upper coastal plains. The Gulf and Atlantic Coastal Plains The sedi- soils generally have sandy A horizons and low ments of the Gulf and Atlantic Coastal Plains are natural fertility. Clay translocation has produced of marine origin. They were derived from the Bt horizons in most of the soils. Many of the sub- weathered products of the adjacent highlands that soils are deficient in calcium for good root were deposited when the area was submerged growth. below the sea. Where calcareous clays rich in smectite were deposited, soils developed with high clay content and great expansion and con- Central Lowlands Most of the Central Low- traction during alternating wet and dry seasons. lands (see Figure 16.3) was glaciated, with the Soils have developed in this kind of parent mate- Ohio and Missouri rivers forming the approximate rial in Texas, Mississippi, and Alabama. Proper- southern boundary of the glaciation. Four major ties of the soils are the result of the dominant advances of the ice occurred during a period of effect of the parent materials. about 1.5 million years. Each advance of the ice

FIGURE 16.5 Alluvial sediments, deposited along streams and rivers, are common parent materials for soil formation throughout the world. 256 SOIL GENESIS was followed by a long interglacial period. The states, most of the glacial sediments have a high glacial and interglacial periods and approximate sand content. times of their occurrence are listed in Table 16.2. During deglaciation, there was rapid ice melt- During deglaciation, rivers and streams of water i ng in summer and large rivers carried away the flowed from the melting ice and carried and de- melt water. As a result, stream valleys with very posited material called outwash near the ice wide floodplains were produced on which sedi- front. Outwash consists of coarse-textured sediments accumulated. During the winter, there was ments. Clay, and to a lesser extent silt, were little meltwater, and the sediments on the flood- carried by water into glacial lakes, where the plains were exposed to the winds. The floodplain clay settled to form calcareous, fine-textured materials dried, and in the absence of a vegetative lacustrine (lake) sediments. The melting of ice cover, the silt-sized particles were preferentially produced moraine sediments by the direct depo- picked up by westerly winds and deposited along sition of material in the ice onto the ground sur- the eastern sides of the major rivers in the degla- face. The moraine sediments are unsorted and are ciated area. These sediments contain mainly silt, called till. Sometimes the ice front readvanced with smaller amounts of clay, and are called over previously deposited sediments. As a result loess. The loess is thickest near the river and of these varied activities, glacial parent materials decreases in thickness with distance away from are very heterogenous, variable in composition, the river. In some cases, the loess was more than and commonly stratified, as shown in Figure 16.6. 100 feet thick. Loess about 50 feet thick along the The nature of the sediments was affected by the Mississippi River in western Tennessee is shown kinds of rocks the glacier moved over and the in Figure 16.7. types of materials that were incorporated into the Loess is the dominant parent material in the ice. As a result, some glacial materials contain soils of Iowa and Illinois. The high silt content some boulders and stones, but most are com- of the loess parent material is reflected in the posed of highly variable amounts of sand, silt, and abundance of silt loam A horizons and the high clay. Another important component was calcium water-holding capacity and fertility of many carbonate whose content was related to the extent of the Corn Belt soils. Many soils beyond the so- to which the glacier moved over limestone and called loess area have been influenced by a thin other calcareous rocks. In the northern part of layer of loess. In some of these soils, the up- Wisconsin, Michigan, and the New England per soil horizons have developed in loess and the lower horizons have developed in till or limestone. Much loess exists outside of the glaciated part TABLE 16.2 Glacial and Interglacial Ages in of the Central Lowlands and is found in the Inte- North America rior Plains, especially in Nebraska and Kansas. This is an area of limited precipitation and steppe (short grass) vegetation. The loess is believed to have resulted from transport of silt from a source area located to the west and under conditions of limited protection of land by a vegetative cover. Extensive areas of nonglacial loess also occur in China and Argentina. Thick loess overlies hills of basalt in the Palouse area of eastern Washington and western Idaho. The loess originated from the deserts of central Washington. About 10 percent

ROLE OF PARENT MATERIAL IN SOIL GENESIS 257

FIGURE 16.6 Sand deposited by water and later overridden by glacial ice,which melted and left a layer of till; an example of stratified parent material.

of the worlds land has been influenced by loess Plains. After the Rocky Mountains were formed, deposition. erosion and subsequent sedimentation produced sediments along the eastern side of the moun- Ineio Plain The Interior Plains region is tains in a manner analogous to the formation of comparable to the western part of the Great the Coastal Plains sediments east and south of

FIGURE 16.7 A loess deposit about 50 feet (15 ms) thick on the uplands along the eastern side of the Mississippi River floodplain in western Tennessee. 258 SOIL GENESIS the Appalachian Mountains. These sediments, Ash is widely distributed by winds and is a however, contain more weatherable minerals, minor but frequent addition to many soils a great and the resulting soils are much more fertile than distance from the ash source. Ash is a significant those formed from the coastal plains sands. There component of the loess in the Palouse in the are considerable areas of sand and, in some northwestern United States and on the Pampa in places, erosion has exposed older sediments un- Argentina. In some instances, an occasional addi- derneath. Glacial-derived parent materials cover tion of ash significantly increases the fertility of the Interior Plains region north of the Missouri highly weathered soils as occurs on the Serengeti River. Plain in Kenya, Africa.

Basin and Range Region The Basin and Range Effect of Parent Material Properties on region consists of a series of low mountains sepa- Soil Genesis rated by broad basins. The area was subjected to Horizon development, especially many B hori- block-faulting with the ranges representing the zons, depends on the translocation of fine-sized uplifted areas and the down-faulted blocks repre- particles in water. A parent material composed of senting the basins. Weathering and erosion have 100 percent quartz sand would not weather to produced sediments of coalescing alluvial fans, produce mineral colloidal particles. In such par- which fill most of the basins. Lacustrine sedi- ent material, little else can happen than some ments are of minor extent. organic matter accumulation due to plant growth. Early in this chapter, it was noted that a sand- textured parent material in a humid environment led to the development of Bhs horizons. Within Volcanic Ash Sediments Volcanic ash distri- the parent material there was a small amount of bution parallels that of volcanic mountains, primary minerals that weathered to release some which were the source of the ash. The 1980 erup- iron and aluminum. There was insufficient clay to tion of Mount St. Helens brought a new awareness form Bt horizons, however. When soils develop in of the existence and importance of volcanic ash in a parent material that is essentially all clay, little the northwestern part of United States. Though water will migrate downward to transport parti- much of the material ejected in 1980 was crystal- cles. Parent materials of intermediate texture lead line in nature, ash is commonly amorphous and to the development of a wide variety of soils. weathers to form amorphous allophane and iron The texture and structure of a soil will affect and aluminum oxides. Allophane has a high pH- soil genesis by affecting water infiltration and ero- dependent cation exchange capacity. Soils with sion, permeability and translocation of materials amorphous minerals lack the distinct sand and in water, protection and accumulation of organic silt particles found in most mineral soils. The matter, and solum (A plus B horizon) thickness. major agricultural area in the United States where The thicker solum of the Spinks soil, as compared soils have developed from ash occurs on Hawaii, with the St. Clair soil, is shown in Figure 16.9. The the large island of the Hawaiian chain, where thicker solum is due to the lower clay content, sugarcane is the dominant crop. Although ash is a greater water permeability, and greater downward minor parent material near Honolulu, Hawaii, on migration of clay before lodgement during the the island of Oahu, the famous landmark, Dia- formation of the Bt horizon in the Spinks soil. mond Head, is an old volcanic cone composed of Greater permeability of sandy material also con- consolidated ash, as shown in Figure 16.8. tributes to the more rapid removal of lime (cal-

ROLE OF PARENT MATERIAL IN SOIL GENESIS 259

FIGURE 16.8 Diamond Head is a famous landmark near Honolulu, Hawaii, and is an old volcanic cone composed of consolidated layers of . ash (tuff). Note that the cone is highest on the right because the prevailing winds were from the left during ash fall.

cium carbonate), resulting in more rapid develop- Stratified Parent Materials ment of acidity and a greater leaching depth of the A stratified parent material is shown in Figure lime. Note that the texture of the A and B horizons 16.6. A layer of loess over till was shown to be a in Figure 16.9 is related to the clay content of the C condition found in the glaciated part of the Cen- horizon (parent material). tral Lowlands. Frequently, the A or the A and B

FIGURE 16.9 Relationship between texture of the parent material and the thickness and texture of horizons of three forest soils in the north- central part of the United States. 260 SOIL GENESIS horizons develop in the loess and the lower part of because of water saturation and anoxia, low tem- the soil develops from till. Soil permeability to perature, or acidity, an abundance of organic mat- water (hydraulic conductivity) changes at the ter may accumulate. Sometimes, this results in a boundary of materials of different texture, which mat of organic matter on the surface of mineral affects the downward movement of water. Alluvial soil. In permanently ponded areas throughout the parent materials typically are stratified. Where world, thick deposits of peat and muck have streams dissect the landscape, various materials formed. The Florida Everglades is one of the best- may be exposed for soil formation, giving rise to a known areas. Organic deposits are the parent ma- series of soils whose differences are due mainly to terial for organic soils. parent material differences. Such a sequence of soils is a lithosequence, as shown in Figure 16.10. In soils with horizons developed from differ- ROLE OF CLIMATE IN SOIL GENESIS ent parent materials, numbers are used to indicate this feature. For example, a soil with A and E Climate greatly affects the rate of soil genesis. In horizons developed in loess, and Bt and C hori- areas that are permanently dry and/or frozen, soil zons developed in till has a horizon sequence of does not form. The two components of climate to A-E-2Bt-2C. The 2 indicates that the Bt and C hori- be considered are precipitation and temperature. zons developed in a different or second parent material. Precipitation Effects Water is necessary for mineral weathering and plant growth. Water in excess of field capacity Parent Material of Organic Soils (surplus water) participates in the downward In locations where considerable quantities of translocation of colloidal particles and soluble plant material grow and where decay is limited salts. The limited supply of water in deserts re-

FIGURE 16.10 A roadcut showing outwash overlying sandstone. Strata of different materials can produce a lithosequence of soils on a hillside where the materials are exposed, as shown in the left side of the photograph. ROLE OF CLIMATE IN SOIL GENESIS 26 1 suits in soils that tend to be alkaline, relatively The symbol k is used to indicate this calcium unweathered, low in clay and organic matter con- carbonate accumulation feature. This feature is tent, and cation exchange capacity. Generally, shown in Figure 16.11 for a soil that has a Ck soils of the arid and subhumid regions tend to be horizon. Soils in deserts commonly have k hor- fertile, except for the limited ability of the soil izons. microbes to mineralize soil organic matter and to Increases in precipitation have been posi- supply available nitrogen. Where there is suffi- tively related with greater: (1) leaching of lime and cient water for only limited leaching, the carbo- greater depth to a k layer, (2) development of soil nates tend to be moved downward only a short acidity, (3) weathering and clay content, and (4) distance, where they accumulate and form a zone plant growth and organic matter content. A study or horizon of calcium carbonate accumulation. of surface soils developed in loess along a tra- verse from Colorado to Ohio showed that as precipitation increased, the depth to the k layer increased until the annual precipitation was about FIGURE 16.11 Grassland soil developed under subhumid climate. Limited leaching is expressed by 26 inches (65 cm). This occurred near the the presence of a k layer (accumulation of Nebraska-Iowa border. At this location, pH of A carbonates) in the upper part of the C horizon horizons was about 7. Eastward along the tra- between depths of 70 and 110 centimeters. verse, sufficient leaching occurred to move cal- (Photograph courtesy USDA.) cium carbonate to the water table, and k layers were generally absent. In addition, the surface soils became more acid and those in Ohio had a pH of about 5. Increasing precipitation from eastern Colorado to Indiana is associated with a change in native vegetation. The annual precipitation increases from about 16 to 36 inches (40 to 90 cm). The vegetation changes from widely spaced, bunch and short grasses in Colorado, to short and tall grasses in Nebraska, and to tall grasses in Illinois and Indiana. These changes are associated with an increase in organic matter from 180 to 360 metric tons per hectare (80 to 160 tons per acre) to a depth of 100 centimeters (see Figure 16.12). Similar changes in organic matter content occur on the grasslands in the Pampa of Ar- gentina. Along these traverses with increasing precipitation, increasing soil organic matter contents of well-drained upland soils are associated with greater annual production of the grasses. East of the grassland area the vegetation changes to forest, and the soil organic matter content is considerably less. The organic matter content of tall grass soils in Illinois decreases from 360 metric tons per hectare to 150 metric tons per hectare for forest soils in Ohio (see Figure 16.12). 262 SOIL GENESIS

FIGURE 16.12 Generalized map showing organic matter content of soils, in metric tons per hectare 100 centimeters, as related to climate and vegetation. (Adapted from Schreiner and Brown, 1938.)

Increasing precipitation in the tropics also soils of tundra regions are rich in organic matter results in increasing amounts of organic matter in even though plant growth rates are very slow. tropical soils. The relationship between grassland Even slower are the rates of microbial decom- and forest soils in the temperate region, however, position of organic matter due to both low temper- is reversed in the tropics. The forested, humid, ature and soil wetness. The grassland soils of the tropical soils have a greater organic matter con- Great Plains have a gradual decrease in organic tent than the soils of the adjacent drier grassland- matter with increasing annual temperature. (see shrub savannas. It has been suggested that high Figure 9.4). acidity and infertility of savanna soils are partially The forested soils of the southeastern United responsible for the lower production of plant ma- States are shown to contain only 60 percent as terial on the tropical savannas than that in temper- much organic matter as do the forested soils fur- ate grasslands. ther north (see Figure 16.12). This difference is partially due to the generally more sandy nature of the A and E horizons of the southern soils on the Temperature Effects coastal plains as compared with the finer-texture Every 10°C increase in temperature increases the of the forest soils further north. rate of chemical reactions approximately two Based on the observations of the relation be- times. Increased weathering and clay formation tween temperature and total soil organic matter occur with an average increase in soil temper- content in the eastern United States, one would ature. expect tropical soils to be low in organic matter. The relationship between average tempera- Tropical rain forests produce more organic matter ture and plant growth and the accumulation of and have greater organic matter decomposition organic matter is complex. The organic matter than temperate region forests, because of year- content of soil is the net result of plant growth or round favorable temperature and precipitation. the addition of organic matter, the rate of organic Temperate region forests have a long dormant matter mineralization, and the soil's capacity to winter period, resulting in less plant growth and protect organic matter from mineralization. Many decomposition of organic matter than that in trop- ROLE OF ORGANISMS IN SOIL GENESIS 263 ical rain forests. Therefore, the content of soil study of forest and grassland soils developed from organic matter tends to be similar in the temper- similar parent material in southern Wisconsin re- ate forests and tropical rain forests. Although revealed that grassland soils contain about twice as searchers have found slightly more organic matter much total organic matter as forest soils. In addi- in the tropical counterparts, the differences are of tion there is a marked difference in the distribu- little or no importance. The greater dominance of tion of organic matter within the soil profile (see amorphous clays in tropical soils may also be a Figure 16.13). factor in the protection of organic matter. The explanation for the differences in the amount and distribution of organic matter is related to the differences in the growth habits of the Climate Change and Soil Properties two kinds of plants. The roots of grasses are short- Some soils have been influenced by more than lived and each year contribute to the soil large one climate because of climate shifts. A study of amounts of organic matter that become humified. buried soils, paleosols, reveals the nature of the Furthermore, there is a gradual decrease in root climate that existed many years ago when ancient density with increasing soil depth, which parallels soils formed. In the southwestern United States the gradually decreasing organic matter content. the uplift of the Rocky Mountains cut off moisture- In the forest, by contrast, roots are long-lived and laden winds and created a drier climate east of the the annual addition of plant residues is largely as mountains. Today, some of the soils on the south- leaves and dead wood that fall directly onto the western deserts have a strongly developed Bt hori- soil surface. Some of the plant materials decom- zon, which indicates that a humid climate existed pose on the soil surface, and small animals trans- there when the Bt horizon was formed. port and mix some of the organic matter with a relatively thin layer of topsoil. In the hardwood forest of southern Wisconsin earthworms were ROLE OF ORGANISMS IN SOIL GENESIS active and 81 metric tons of organic matter was in Plants affect soil genesis by the production of the top 6-inch layer per hectare and only 25 metric organic matter, nutrient cycling, and the move- tons in the next deeper 6-inch layer. Thus, in the ment of water through the hydrologic cycle. Mi- forest soil 47 percent of the soil organic matter croorganisms play an important role in organic was in the top 6 inches as compared to 35 percent matter mineralization and the formation of hu- for the grassland soil. The dark-colored surface mus. Soil animals are consumers and decom- layer enriched with organic matter, the A horizon, posers of organic matter; however, the most obvi- is thicker in the grassland soil than in the forest ous role of animals appears to be that of earth soil. movers. Another interesting feature shown in Figure A most obvious effect of organisms on soil 16.13 is the similar amount of total organic matter genesis is that caused by whether the natural veg- in each ecosystem. In the forest, however, most of etation is trees or grass. The following discussion the organic matter is found in the standing trees. will emphasize the differential effects of trees and When settlers cleared and burned the trees, more grass on soil genesis. than one half of the organic matter in the forest ecosystem was destroyed. By contrast the conversion of the Prairie to use for cropping resulted in Trees Versus Grass and Organic little loss of organic matter. In addition, erosion of Matter Content the topsoil was more detrimental to forest soils Figure 16.12 shows that the forested soils of Ohio than to grassland soils, because of the relatively and Wisconsin contain significantly less organic greater concentration of organic matter near the matter than grassland soils located to the west. A surface of the soil. 264 SOIL GENESIS

FIGURE 16.13 Distribution of organic matter in forest and grassland ecosystems in south-central Wisconsin in metric tons per hectare. (Adapted from Nielson and Hole, 1963, and used by courtesy of F. D. Hole, Soil Survey Division of the Wisconsin Geological and Natural History Survey, University of Wisconsin.)

Vegetation Effects on Leaching through the addition of organic matter. The effect and Eluviation will be to delay their removal from the cation Wide differences in the uptake of ions and, conse- exchange sites and to delay the development of quently, in chemical composition of plants have acidity and the accumulation of exchangeable been well documented. These differences play a aluminum and hydrogen. The data in Table 16.3 role in soil development. Species that normally support the fact that hardwoods maintain a higher absorb large amounts of the cations calcium, pH than do spruce trees when grown on parent magnesium, potassium, and sodium will delay material with the same mineralogical compo- the development of soil acidity because they recy- sition. cle more of these cations to the soil surface Under the same climatic conditions, where for- ROLE OF TOPOGRAPHY IN SOIL GENESIS 265

face of the soil. In some tropical soils that are more than a million years old, and that have had significant termite activity, the soil, in a sense, has been turned over many times. This is one proba- ble cause for the uniformity in properties in many intensively weathered soils in the tropics. For additional consideration of soil animals, see Chap- ter 8.

ROLE OF TOPOGRAPHY IN SOIL GENESIS Topography is used here to refer to the configura- Adapted from R. Muckenhirn, 1949. tion of the surface of a local or relatively small area of land. Differences in topography can cause wide variations in soils within the confines of a ests and grasslands soils exist side by side and single field. Topography determines the local dis- have comparable parent material and slope, for- tribution or disposal of the precipitation and de- est soils show a greater leaching of exchangeable termines the extent to which water tables influ- calcium, magnesium, potassium, and sodium ence soil genesis. Water-permeable soils on and greater eluviation of clay from the A to B broad, level areas receive and infiltrate almost all horizon. Forest soils are more acid than grassland of the precipitation. Soils on sloping areas infil- soils, due to greater leaching of basic cations; trate less than the normal precipitation; hence, they also have less clay in the A horizon and more there is runoff. Depressions and low areas receive clay in the B horizon. A comparative study of soils additional water, making more water available for on 2 to 8 percent slopes in Iowa found that the B/A soil genesis than the normal precipitation. The clay ratio (percent clay in A horizon divided by effects of water relations on soil genesis are more percent clay in B horizon) of the forest soil was 2.0 pronounced in humid than in arid regions. compared with 1.04 for the grassland soil. Clay- pans develop in forest soils in less time than is required in grassland soils. The discussion of the Effect of Slope effect of trees versus grass on soil genesis shows Both the length and steepness of slope affect soil that the same fundamental processes occur in genesis. As the steepness of slope increases, both kinds of soils but differ in degree of expres- there is greater water runoff and soil erosion. The sion. Soils developed under trees and grass be- net effect is a retardation of soil genesis. Gener- come strikingly similar when both have devel- ally, an increase in slope gradient is associated oped impermeable clay-pan subsoils. with less plant growth and organic matter content, less weathering and clay formation, and less leaching and eluviation. Consequently, soils have Role of Animals in Soil Genesis thinner sola and are less well developed on The role of animals in incorporating organic mat- steeper slopes. Even on the native prairies of the ter into the soil surface has been mentioned. Go- central United States, some grassland soils on phers and prairie dogs are well known for their steep slopes have thin A horizons. The major burrowing and nest-building activities. These cause of different soil colors in many fields is soil animals transport and deposit material on the sur- erosion. On eroding slopes the removal of soil 266 SOIL GENESIS produces light-colored soils and the sedimenta- sented by the Gritney and Norfolk soils. Poorly tion and deposition of the eroded soil; where drained soils tend to occur in the low parts of the changes in slope occur, dark-colored soils are landscape where water tables exist close enough produced (see Figure 16.14). to the soil surface to cause various degrees of Many soils on sloping land are in equilibrium in anoxia and reduction. This landscape position is terms of erosion rate and rate of horizon forma- represented by the Bibb soils in Figure 16.15. The tion. As erosion reduces the thickness of the A Rains soils are poorly drained because of a nearly horizon, the upper part of the B horizon is incor- level or slightly depressional position on a broad porated into the lower part of the A horizon. The upland area. The Goldsboro soils (Figure 16.15) upper part of the C horizon is slowly incorporated are intermediate, being moderately-well drained into the lower part of the B horizon. Under these or somewhat poorly drained. conditions, the soil essentially maintains its char- Poorly drained soils have an A horizon that is acter over a long period of time as the landscape usually dark-colored because of the high organic evolves. matter content; the subsoil tends to be gray- colored. Well-drained soils have lighter colored A horizons and have uniformly bright-colored sub- Effects of Water Tables and Drainage soils. Intermediate drainage conditions produce For this discussion, it is assumed that soils are soils with intermediate properties. Whenever the permeable to water, so that landscape position soil is water saturated, downward translocation of plays an important role in affecting the depth to clay and removal of soluble materials are in- the water table and soil drainage. Drainage is a hibited. measure of the tendency of water to leave the soil. As a result of differences in topography, soils Well-drained soils occur on slopes where the wa- developed from similar parent materials may vary ter table is far below the soil surface. The well- greatly within a small area. A sequence of soils drained soils shown in Figure 16.15 are repre- along a transect that differs because of topogra-

FIGURE 16.14 Cultivated land showing light-colored soil on actively eroding slopes and dark-colored soils where eroded material has been deposited and is accumulating. ROLE OF TOPOGRAPHY IN SOIL GENESIS 267

FIGURE 16.15 Representa- tive landscape showing the relative location of some important soils and depth to the seasonal high water table in Wilson County, North Carolina. (From Wilson County, North Carolina, soil survey report, USDA.)

phy is a soil catena, or toposequence. Locally, lands; (2) young soils on upper slopes that devel- variations in soils are due primarily to variations oped in the paleosol; and (3) young (less devel- of topography and parent material. oped) soils that developed from unweathered till on the lower valley slopes. In stream valleys, soils are very young and have developed from recent Topograph, Parent Material, and alluvium and colluvium. Within the landscape Time Interactions there are four major kinds of parent material (each On slopes, water runoff and soil erosion occur of different age) and about six different topo- while simultaneously run-on water and soil depo- graphic positions, as well as some microclimatic sition occur on areas at a lower elevation. Thus, in differences. In such a landscape in southern lo- a landscape, young soils tend to occur where both wa,these soil-forming factors result in formation sedimentation and erosion are active. The oldest of 11 major kinds of soils, as shown in Fig. 16.17. soils tend to occur on broad upland areas where neither erosion nor sedimentation occurs. About 500,000 or more years ago in the north- Uniqueness of Soils Developed in central United States, the Kansan glacier melted, Alluvial Parent Material leaving a relatively flat land surface composed of Weathering typically is more intense near the soil till and other glacial materials. During a long inter- surface than in underlying layers, causing the glacial period, an old, well-developed soil with mineral material to be less weathered with in- high clay content formed in the surface of the till, creasing soil depth. For alluvial soils, the surface as shown in Figure 16.16a. Loess was later depos- horizon is the least weathered because it forms ited on the soil developed from till, which was from the most recently deposited alluvium. buried and became a paleosol (see Figure Alluvium accumulates along the Nile River at 16.16b). Erosion then dissected the till plain and the rate of about 1 millimeter annually, resulting removed the loess from the sloping areas, which in a meter of alluvium every 1,000 years. It is reexposed the paleosol on the upper slopes. Ero- common for alluvial sediments to be more than sion and dissection also exposed unweathered till several hundred meters thick. After subsoil mate- on the lower slopes in the valleys (see Figure rial is buried by more recent alluvium, the subsoil 16.16c). Today, the result is: (1) soils of moderate receives very little additional organic matter from age formed in loess on broad, nearly level up- root growth and the organic matter present con- 268 SOIL GENESIS

FIGURE 16.18 Earth-moving activities of humans to construct terraces on slopes for paddy rice production have resulted in great modification of the natural soils.

tinues to mineralize. This results in a gradual de-

(c) Modern soil Landscape crease in soil organic matter with increasing depth. Alluvial soils tend to be young soils, and FIGURE 16.16 Steps in the formation of a soil landscape in southern Iowa. (Adapted from the horizons are caused mainly by differences in Oschwald et al., 1965.) the properties of the layers of alluvium.

FIGURE 16.17 Relationship of slope, vegetation, and parent material to soils of the Adair-Grundy-Haig soil association in south-central Iowa. (From Oschwald et al., 1965.) REFERENCES 269

On floodplains, sand and the coarser particles from the parent material, and soil properties be- are deposited near the river and may form a levee. come more closely related to the changes that Further from the river, the silt and clay tend to be occurred during soil genesis. The age of a soil is deposited from slower-moving and, in some expressed by the degree of development and not cases, very stagnant water. The result is a general the number of years. In one sense, soils have a life increase in the clay content of soils with increas- cycle that is represented by soils expressing dif- ing distance from the river. ferent degrees of development. When rivers cause rapid down cutting, alluvium Some soils form in parent material derived from at the outer edges of the floodplain may become the direct weathering of rocks. Most soils have elevated relative to the river, so that they no longer formed in sediments. flood. This results in the formation of terraces, Soil genesis, in general, is accelerated by in- and in the absence of flooding and deposition, the creases in precipitation and temperature. soils develop normally with time. Soil genesis is accelerated by tree rather than grassland vegetation. Similar processes occur un- HUMAN BEINGS AS A der both types of vegetation, and old soils are SOIL-FORMING FACTOR similar when developed under trees or grass. Grassland soils have much greater content of or- One of the most obvious and early effects of hu- ganic matter within the soil than forest soils, and man activity on soils has occurred at campsites the organic matter content decreases less rapidly and villages. Organic residues have been added with increasing soil depth. to the soils from food preparation and preser- Topography affects soil genesis mainly by dis- vation, together with other materials such as persal of precipitation, erosion, and accompany- bones and shells. Human excrement and other ing deposition of eroded material, and soil materials have similarly contributed to the forma- drainage. Youthful soils tend to occur where soil tion of thick, dark-colored layers greatly enriched erosion and soil deposition are active. with phosphorus. Human beings have affected many soils at habi- The use of land for agriculture, forestry, graz- tation sites and through agriculture, waste dis- i ng, and urbanization has produced extensive posal, and earth moving. changes in soils. Many of these changes have been discussed, including soil erosion, drainage, salinity development, depletion and addition of organic matter and nutrients, compaction, and REFERENCES flooding. Sometimes, solid waste disposal pro- Aandahl, A. R. 1948. "The Characterization of Slope duces new soils that are little more than an accu- Positions and Their Influence on the Total Nitrogen mulation of trash. Millions of acres of land have Content of a Few Virgin Soils of Western Iowa." Soil soils with properties that are due more to human Sci. Soc. Am. Proc. 13:449-454. activities than to soil-forming factors. In China, Bidwell, 0. W. and F. D. Hole. 1965. "Man as a Factor in land has essentially been created to grow food Soil Formation." Soil Sci. 99:65-72. crops. The enormous movement of soil and the Blizi, A. F. and E. J. Ciolkosz. 1977. "Time as a Factor in shaping of the landscape for production of paddy the Genesis of Four Soils Developed in Recent Allu- rice is illustrated in Figure 16.18. vium in Pennsylvania." Soil Sci. Soc. Am. J. 41:122- 127. SUMMARY Boul, S. W., F. D. Hole, and R. J. McCracken. 1980. Soil Genesis and Classification, 2nd ed., Iowa State Press, Soils are the products of evolution. Over time, the Ames. soil properties are less affected by inheritance Ericson, D. B., M. Ewing, and G. Wollin. 1964. "The 270 SOIL GENESIS

Pleistocence Epoch in Deep-Sea Sediments." Sci- Agronomy Special Report 42. Iowa State University, ence. 146:723-732. Ames. Fanning, D. S. and M.C.B. Fanning. 1989. Soil Morphol- Ruhe, R. V., R. B. Daniels, and J. G. Cady. 1967. "Land- ogy, Genesis, and Classification. Wiley, New York. scape Evolution and Soil Formation in Southwestern Franzmeier, D. P. and E. P. Whiteside. 1963. A Chro- Iowa." USDA Tech. Bull. 1349, Washington, D.C. nosequence of Podzols in Northern Michigan: I, II, Sanchez, P. A., M. P. Gichuru, and L. B. Katz. 1982. and III. Mich. Agr. Exp. Sta. Quart. Bull. 46:2-57. East "Organic Matter in Major Soils of the Tropical Lansing. and Temperate Regions." 12th Int. Congress Soil Jenny, Hans. 1980. The Soil Resource. Springer-Verlag, Sci. Symposia Papers Vol 1. pp. 101-104, New New York. Delhi. Muckenhirn, R. J., E. P. Whiteside, E. H. Templin, R. F. Schreiner, O. and B. E. Brown. 1938. "Soil Nitrogen." Chandler, and L. T. Alexander. 1949. "Soil Classifi- Soils and Men, USDA Yearbook of Agriculture, Wash- cation and the Genetic Factors of Soil Formation." ington, D.C. Soil Sci. 67:93-105. Twenhofel, W. H. 1939. "The Cost of Soil in Rock and Nielsen, G. A. and F. D. Hole. 1963. "A Study of the Time." Am. J. Sci. 237:771-780. Natural Processes of Incorporation of Organic Matter Vance, G. F., D. L. Mokma, and S. A. Boyd. 1986. "Phe- into Soil in the University of Wisconsin Arboretum." nolic Compounds in Soils of Hydrosequences and Wis. Acad. Sci. 52:213-227. Developmental Sequences of Spodosols." Soil Sci. Oschwald, W. R. et al. 1965. "Principal Soils of Iowa." Soc. Am. J. 50:992-996. CHAPTER 17

SOIL TAXONOMY

Classification schemes of natural objects seek to diagnostic epipedons that differ in thickness, organize knowledge, so that the properties and color, structure, content of organic matter and relationships of these objects may be remem- phosphorus, mineralogy, and the degree of bered and understood for some specific purpose. leaching. Many soil classification schemes have been developed, and most have been on a national basis. In 1975, the United States Department of Agricul- ture, with the help of soil scientists from many Mollic Horizon nations, published Soil Taxonomy. Many of its The word mollic is derived from mollify, which terms and ideas also appear in the legend of the means to soften. The mollic horizon is a surface world soil map published by the Food and Agri- horizon that is soft rather than hard and massive culture Organization (FAO) of the United Nations. when dry. Mollic epipedon formation is favored Knowledge of either system makes use of the where numerous grass roots permeate the soil other system possible with minimal adjustments. and a moderate to strong grade of structure is created. Except for special cases, mollic horizons are dark-colored, contain at least 1 percent organic matter, and are at least 18 centimeters thick. They are only minimally or moderately weathered DIAGNOSTIC SURFACE HORIZONS and leached. The cation exchange capacity is 50 Diagnostic horizons have unique properties and percent or more saturated with calcium, magne- are used with other properties to classify soils. sium, potassium, and sodium when the cation Diagnostic surface horizons (epipedons) form at exchange capacity is determined at pH 7. The the soil surface and diagnostic subsurface hori- major grassland soils of the world have mollic zons form below the soil surface. There are seven epipedons (see Figure 17.1).

271 272 SOIL TAXONOMY

ruuunw i i. i aon wun a mouic epipeaon.

Umbric and Ochric Horizons histic epipedon. Histic epipedons are 0 horizons. Other diagnostic epipedons can be related to the A histic epipedon must be water saturated for at mollic horizon, as shown in Figure 17.2. The um- least 30 consecutive days during the year, unless bric epipedon is similar to the mollic in overall the soil has been drained. The degree of decom- appearance of thickness and color; however, it is position of the organic matter in histic horizons is more leached and has a saturation with basic indicated with the following symbols: i for fibric, e cations calcium, magnesium, potassium, and so- for hemic, and a for sapric. Fibric (i) is the least dium that is less than 50 percent when the cation decomposed and contains a large amount of rec- exchange capacity is determined at pH 7. Umbric ognizable plant fibers; sapric (a) is the most de- horizons form in some very humid forests, as in composed; and hemic (e) is of intermediate de- the Coastal Range of the northwestern United composition. The relationship of the histic, States. In most forests, the epipedon is thinner horizon to the mollic horizon is shown in Figure than is that of the umbric, is lower in organic 17.2. matter, and is lighter in color. This epipedon is called ochic. Melanic Horizon Melanic epipedons are thick, black colored, and Histic Horizon contain high concentrations of organic matter. Where soil development occurs under conditions The moist Munsell value and chroma are 2 or less, of extreme wetness, as in swamps or lakes, the and the organic carbon content is 6 percent or epipedon is organic in nature and is, typically, a more but less than 25 percent. The organic matter DIAGNOSTIC SUBSURFACE HORIZONS 273

is thought to result from the supply of large richment of soil is a standard technique used by amount of root residues from a graminaceous anthropologists to determine where ancient vegetation in contrast to organic matter that re- campsites, villages, and roads were located. Many sults from a forest vegetation. anthropic horizons have been found in Europe, Melanic horizons also have andic soil proper- and some have been found at old presettlement ties. Andic soil properties result mainly from the campsites in the United States. Some anthropic weathering of volcanic materials, which contain a horizons in the Tombigee River valley in northern significant amount of volcanic glass (noncrystall- Mississippi are more than a meter thick, contain ine ash, pumice, lava, etc). The dominant clay 0.6 to 3 percent organic carbon, and 1 to 5 percent minerals include allophane and some oxides of charcoal fragments. In the deeper parts of the iron and aluminum. Recent work with electron anthropic horizons the carbon had been dated as microscopy, and other techniques, has shown being 7,000 years old. The citrate soluble P 2 0 5 that these minerals are characterized by an or- content of the anthropic horizons exceeded 250 derly arrangement of atoms. But the orderly ar- ppm compared with 80 and 20 ppm in the surface rangement is short range and does not in a regular and subsoils, respectively, of adjacent soils. manner form a 3-dimensional pattern like that The plaggen epipedon is found in Europe found in crystalline minerals such as vermiculite where long-continued manuring has produced a or kaolinite. The particles are very small with an surface layer 50 centimeters or more thick. During enormous surface area, which results in high the Middle Ages, farmers in northwestern Europe anion exchange capacity and a large capacity to collected sods from forests and heath lands complex organic matter. These minerals are con- where soils were very sandy. These clumps of sod sidered to be amorphous or short-range minerals. contained sand and were used for bedding in Melanic horizons are black and have high or- barns. The manure from the barn was applied to ganic matter content similar to some histic hori- the land, which resulted in the slow accumulation zons. Even though the organic matter content is of a thick, sandy epipedon enriched with organic high, the organic matter in melanic horizons is matter. In some places, a 1-meter thick plaggen mostly associated or complexed with minerals, epipedon was created in 1,000 years. Plaggen hor- which results in properties dominated by the min- izons typically contain artifacts, such as bits of eral fraction rather than the organic fraction. The brick or pottery, and today these soils are valued horizon is also characterized by low bulk density for agricultural use. and high anion exchange capacity. The derivation of the the epipedon names and their major features are given in Table 17.1.

A P H DIAGNOSTIC SUBSURFACE HORIZONS The anthropic and plaggen horizons are formed by human activity. The anthropic epipedon re- Generally, diagnostic subsurface horizons are B sembles the mollic horizon in color, structure, horizons. They may form immediately below a and organic matter content; however, the phos- layer of leaf litter, as in the case of the E horizon. phorus content is 250 or more ppm of P 2 0 5 equiv- More than 15 subsurface diagnostic horizons have alent soluble in 1 percent citric acid solution. The been recognized. anthropic horizon occurs where human activity resulted in the disposal of bones and other refuse near places of residence, or in agriculture at sites C H of long-continued use of soil for irrigated. The cambic horizon can form quickly, relatively cropping. Measurement of the phosphorus en- speaking, because changes in the color and struc- 274 SOIL TAXONOMY

ture, and some leaching will convert the subsoil more than 40 percent clay, the illuvial horizon parent material into a cambic horizon. Cambic must have at least 8 percent more clay to qualify horizons are not illuvial horizons and, generally, as an argillic horizon. Argillic horizons usually they are not extremely weathered. A cambic hori- require several thousands of years to form, as zon is a Bw horizon. A parent material with a shown in Figure 2.7. texture of fine sand (or even finer) is required for A natric horizon is a special kind of argillic the development of cambic horizons. horizon. These horizons have typically been affected by soluble salts. In addition to the properties of the argillic horizon, natric horizons com- A N H monly have a columnar structure and a sodium The gradual illuvial accumulation of clay in a adsorption ratio (SAR) of 13 or more (or 15 per- cambic horizon converts a cambic horizon (Bw cent or more exchangeable sodium saturation). horizon) into a Bt horizon. Typically, clay skins The natric horizon is a Btn horizon; the n indicates occur on the ped surfaces of Bt horizons. When the accumulation of exchangeable sodium. An the ratio of clay in the Bt horizon, compared with example of a natric horizon is shown in Figure that of the overlying eluvial horizon, is 1.2 or 17.3. more, the Bt horizon becomes an argillic horizon, in which the overlying eluvial horizon contains 15 to 40 percent clay. If the eluvial horizon has less K H than 15 percent clay, there must be at least 3 In intensively weathered soils, the genesis of sub- percent more clay in the illuvial horizon to qualify soil layers that have more clay than the overlying A as an argillic horizon. If the eluvial horizon has horizon is obscure because of the very long time DIAGNOSTIC SUBSURFACE HORIZONS 275

lation of sesquioxides, humus, or humus plus sesquioxides, respectively. Evolution of the spodic horizon is summarized in Table 16.1.

A H The loss by eluviation of sesquioxides and clay, during the formation of spodic and argillic horizons, tends to leave behind a light-colored overlying eluvial horizon called the albic horizon. Albic is derived from the word white. The horizon is eluviated and is labeled an E horizon. The color of the albic horizon is due to the color of the primary sand and silt particles rather than to the particle FIGURE 17.3 Soil with a natric horizon. The upper coatings. Albic horizons are commonly underlain part of the natric horizon is at 20 centimeters and is by spodic, argillic, or kandic horizons. characterized by whitish-topped columnar peds.

O H of soil formation. Subsoils with much higher clay The oxic horizon (Bo horizon) is a subsurface content than overlying horizons may show no evi- horizon at least 30 centimeters thick that is in an dence of clay skins. Therefore, many tropical soils advanced stage of weathering. It is not dependent have a subsoil horizon similar to the argillic hori- on a difference in the clay content of subsoil ver- zon, except that the clay minerals have very low sus the topsoil horizons. Oxic is derived from the cation exchange capacity, which is indicative of word oxide. Oxic horizons consist of a mixture of lo acii cla. The clay minerals are predomi- iron and/or aluminum oxides with variable nately kaolinite and oxidic clays. The cation ex- amounts of kaolinite and highly insoluble ac- change capacity of the clay, determined at pH 7, is cessory minerals such as quartz sand. There are 16 cmol/kg (16 meq/100 g) or less or the ECEC is few if any weatherable minerals to weather and 12 cmol/kg or less. These clay-rich horizons are supply plant nutrients, and the clay fraction has a called kandic. small cation exchange capacity-16 cmol/kg (meq/ 100 g) or less as determined at pH 7 or 12 or less cmol/kg at the soil's natural pH. Soils with S H oxic horizons have essentially reached the end In sand-textured parent material there is little clay point of weathering. for illuviation and rapid water permeability. Under conditions of high precipitation, iron and aluminum oxides (sesquioxides) are released in weath- C, G, S H ering. These oxides complex or chelate with or- The calcic horizon is a horizon of calcium carbo- ganic matter and are transported to the subsoil, nate or calcium and magnesium carbonate accu- resulting in the formation of an illuvial horizon. mulation. Calcic horizons develop in soils in An illuvial subsoil horizon that is greatly enriched which there is limited leaching and the carbo- with these amorphous compounds is a spodic nates are translocated downward. However, the horizon. Spodic horizons can be Bs, Bh, or Bhs carbonates are deposited within the soil profile horizons, depending on the degree of accumu- because there is insufficient water to leach the

276 SOIL TAXONOMY

carbonates to the water table. The symbol k indi- result, the soil moisture regime is based on a cates an accumulation of carbonates, as in Bwk or moisture control section that is represented by the Ck. Gypsic horizons have an accumulation of sec- difference between the depth of soil at the perma- ondary sulfates, and that is indicated with the nent wilting point that is wetted by 2.5 centimeters symbol y. Cemented calcic and gypsic horizons of water as compared with the depth wetted by 7.5 are petrocalcic and petrogypsic horizons, respec- centimeters of water. The moisture control sec- tively. Salic horizons contain a secondary enrich- tion is generally within the 10-centimeters and ment of salts more soluble than gypsic and are 90-centimeters depths, being thicker in sandy indicated with the symbol z. soils and thinner in clay soils. The amount of water in the moisture control section provides an indication of the sufficiency or deficiency of water S D for plant growth and the water status of the soil for W H other uses. Other symbols and meanings used for subordinate distinctions within soil horizons or layers include the following: A M R Soils with aquic moisture regime are wet. At b Buried genetic horizon sometime during the year, the lower soil horizons c Concretions or hard nonconcretionary are saturated. In many instances, the entire soil is nodules saturated part of the year. Water saturation occurs f Permafrost because of the location of the soil in a landscape g Strong gleying (indicating iron reduction) position that is naturally poorly drained or where m Cementation or induration an impermeable layer causes saturation. Typi- q Accumulation of silica cally, the top of the saturated soil fluctuates dur- r Weathered or soft rock ing the year, being deeper during the driest part of v Plinthite (iron-rich reddish material that the year. Artificial drainage is needed to grow dries irreversibly) plants that require an aerated root zone. Soils with x Fragipan character (high density and an aquic moisture regime are unsuitable for build- brittleness) ing sites unless these soils are dewatered with a drainage system. Subsoil colors in mineral soils with aquic regime are frequently gray, which indicates reduc- SOIL MOISTURE REGIMES ing conditions and low oxygen supply. Alternating The soil moisture regime is an expression of the reducing and oxidizing conditions are associated changes in soil water over time. It refers to the with a fluctuating water table that produces mot- presence or absence of either groundwater or of tled colors. The high water content tends to be water held at a potential of less or more than associated with a high content of organic matter -1,500 kPa (-15 bars) by periods of the year. Three and, when properly drained, many aquic soils soil water conditions are recognized: soil that is become very productive agricultural soils. water saturated (wet soil), soil between saturation and the permanent wilting point (moist soil), and soil drier than the wilting point (dry soil). U P M R The water content of the upper soil horizon Udic means humid. Soils with a udic moisture changes frequently throughout the year because regime are not dry in any part of the moisture of repeated rainfall and subsequent drying. As a control section as long as 90 cumulative days in SOIL MOISTURE REGIMES 277 most years. Udic moisture regimes are common have an ustic moisture regime. Native vegetation in soils of humid climates that have well distrib- was mainly mid, short, and bunch grasses. Sur- uted precipitation. The amount of summer rain- plus water is rare, leaching is limited, and soils fall, plus stored water, is approximately equal to tend to be fertile. or exceeds the potential evapotranspiration (PE). In areas where soils have an ustic moisture In other words, if droughts occur, they are short regime, there is enough rainfall in winter, early and infrequent. Forests and tall grass prairies are spring, and summer to produce a significant typical native vegetation. amount of soil water recharge or storage. This In most years there is some surplus water that water, plus the precipitation during the spring and percolates downward and produces leaching. Nu- early summer, is sufficient for the production of a trient ions in the water are leached, removing a wheat crop that ripens before the soil becomes significant amount of plant nutrients from the soil. dry in mid- and late summer. Corn does not ripen Leaching losses of calcium and magnesium may until fall and generally cannot be produced unless be greater than the amount required to produce irrigated. Droughts are common. Sorghum is a an average crop. Udic soils are common from the popular crop because it can interrupt growth East Coast of the United States to the western when water is lacking and grow again if more boundaries of Minnesota, Iowa, Missouri, Ar- rainfall occurs. Cattle grazing is an important kansas, and Louisiana. Old soils with udic mois- land use. ture regime tend to be naturally acid and infertile Soils in monsoon climates have ustic moisture as a result of leaching. An example of soil water regimes because there is enough water during changes over time in a soil with a udic soil mois- the rainy season for significant plant growth; ture regime is shown in Figure 17.4 and the udic however, there is a long dry season with a deficit. moisture regime diagram for Memphis, Tennes- see is given in Figure 5.15. If the regime is characterized by very wet condi- A M R tions, the regime is perudic. In the perudic re- The driest soils have an aridic moisture regime. gime, precipitation exceeds evapotranspiration The subsoil, or moisture control section, is dry every month in most years, water moves through (permanent wilting point or drier) in all parts for the soil every month unless the soil is frozen, and more than half the growing season, and is not there are only occasional periods when some moist (wetter than the permanent wilting point) in stored soil water is used. some parts for as long as 90 consecutive days during the growing season in most years. Most soils with an aridic moisture regime are found in U M R arid or desert regions with widely spaced shrubs The concept of the ustic moisture regime relates and cacti as the native vegetation. A crop cannot to soils with limited available water for plants, but be matured without irrigation. Some climatic data water is present at a time when conditions are and the soilwater balance typical of an aridic suitable for plant growth. Soils with ustic moisture moisture regime are given in Figure 17.4. The regime are dry in some part of the moisture con- precipitation is lower than the potential evapo- trol section for as long as 90 cumulative days in transpiration (PE) during most months of the year. most years. There is moisture available, stored There is very little soil water storage or recharge, soil water plus precipitation, for significant plant which means there is little water retained for plant growth at some time during the year and a deficit growth. The result is a frequent lack of water for part of the year, as shown in Figure 17.4. Most plants and a large water deficit. Many of the native soils on the central and northern Great Plains plants have an unusual capacity to endure very 278 SOIL TAXONOMY

low water potentials in the soil and within their ness during the winter. Leaching and weathering tissues. Grazing is the dominant land use except may occur during the winter even though sum- where irrigation permits crop production. mers are long, very dry, and hot. With limited water storage, and little if any summer rainfall, a large water deficit occurs in summer. Xeric soils X M R are typical of the valleys of California where a Soils in areas where winters are cool and moist large variety of crops, including vines, fruits, and summers are hot and dry have a xeric soil nuts, vegetables, and seeds are produced with moisture regime. This kind of climate is known as irrigation. Xeric soils of the Palouse in Wash- a Mediterranean climate. The precipitation occurs ington and Oregon are used for winter wheat. in the cool months when the evapotranspiration is Here, there is a good match between the time low and surplus water may readily accumulate. when the wheat needs water and when soil water This frequently results in considerable soil wet- is available. CATEGORIES OF SOIL TAXONOMY 279

TABLE 17.2 Definitions and Features of Soil Temperature Regimes

° Frigid soils have warmer summers than cryic soils; both have same MAST. Frigid soils have more than 5° C temperature difference between mean winter and mean summer temperatures at a depth of 50 centimeters (or lithic or paralithic contact, if shallower).

SOIL TEMPERATURE REGIMES CATEGORIES OF SOIL TAXONOMY Soil temperature regimes are defined according The categories of Soil Taxonomy are order, su- to the mean annual soil temperature (MAST) in border, great group, subgroup, family, and series. the root zone (arbitrarily set at 5 to 100 cm). Re- The order is the highest category and the series is gime definitions and some characteristics are the lowest category. given in Table 17.2. Both the soil moisture regimes and the temperature regimes are used to Soil Order classify soils at various categories in Soil Tax- There are 11 orders, each ending in -sol; L. solum, onomy. meaning soil. The orders, together with their deri- 280 SOIL TAXONOMY

TABLE 17.3 Formative Syllables, Derivations, and Meanings of Soil Orders

The Entisol order includes all soils that do not fit into one of the other 10 orders and is composed of soils with great heterogeneity. Extensive treatment of the soil orders can be found in the next chapter. CATEGORIES OF SOIL TAXONOMY 28 1 282 SOIL TAXONOMY

(Table 17.4, conined)

Sbde ad Gea G and their formative elements, meaning, and con- Orders are divided into suborders. The suborder notation are given in Table 17.4. name has two syllables. The first syllable con- For the suborder Aqualfs, aqu is derived from notes something of the diagnostic properties of L. aqua, meaning water, and alf from Alfisol. the soil, and the second syllable is the formative Thus, Aqualfs are Alfisols that are wet; they have element or syllable for the order. Suborder names an aquic soil moisture regime. Boralfs are Alfisols AN EXAMPLE OF CLASSIFICATION: THE ABAC SOILS 283 of the cool regions. Udalfs have an udic moisture consin. More than 16, 000 series have been recog- regime, Ustalfs have an ustic moisture regime, nized in the United States. and Xeralfs have a xeric moisture regime. Note that many of the formative elements of the suborder names connote soil moisture or tempera- AN EXAMPLE OF CLASSIFICATION: ture regimes. The suborder is the most common T ABAC SOILS category used in the discussion of soil geography and in the legends of the soil maps in the next Soils of the Abac series are Typic Ustorthents, chapter. meaning the soils are Entisols (ent), that occur on The legends for the soil maps also use some recent slopes, subject to erosion (orth), have an great group names. In the legend of Figure 18.1 ustic moisture regime (ust), and are typical for the (soil map of the United States), Ala refers to areas Ustorthent great group. The family is loamy, where soils are predominatly Aqualfs with signif- mixed, calcareous, frigid, and shallow. This icant amounts of other soils in decreasing order of means, for example, that the texture is finer than importance. Udalfs are the second most extensive loamy sand (loamy), no one mineral dominates soils, followed by Haplaquepts, which is a great the mineralogy and the soil is calcareous (mixed, group name. Hapl means minimal horizon and calcareous), the mean annual soil temperature is refers to Aquepts that are weakly or minimally between 0 and 8° C (frigid), and there is rock developed. Areas of A2a are dominated by Bor- within 50 centimeter of the soil surface (shallow). alfs, with Udipsamments second in importance. The Abac soil is a loamy, mixed, calcareous, Udipsamments are Psamments (sand-textured frigid, shallow typic Ustorthent. Entisols) with a udic moisture regime. Thus, From this information it can be concluded that some of the formative elements given in Table the soil is not likely to be cultivated and used for 17.4 for suborders are also used to form great cropping. Such soils can produce a moderate group names. Prefixes and their connotations for amount of forage for grazing in the spring and great group names are given in Appendix 3. early summer. The soils occur on steep slopes where it is shallow to rock in Montana. Such soils can be desirable locations for vacations and rec- S, F, S reational activities, as shown in Figure 17.5. The taxonomic categories that follow great group are subgroup, family, and series. The subgroup denotes whether or not the soil is typical (typic) for the great group. Soils that are not typical of the FIGURE 17.5 Landscape in the mountains where many of the soils are-Ustorthents. The soils provide great group are indicated as being intergrades. some pasture for grazing animals; however, the ideal The family category provides groupings of soils summer climate makes such locations desirable for with ranges in texture, mineralogy, temperature, camping, horseback riding, and hiking. and thickness. The primary use of the series name is to relate soils delineated on soil maps to the intrepretations for land use. Series names are taken from the name of a town or landscape feature near the place where the soil series was first recognized. Many of the names are familar, such as Walla Walla in the state of Wash- ington, Molokai in Hawaii, and Santa Fe in New Mexico. The Antigo soil is the state soil in Wis- 284 SOIL TAXONOMY

THE PEDON SUMMARY Soil bodies are large, and hence there is a need for Diagnostic surface and subsurface horizons, to- a smaller soil unit that can be the object of scien- gether with other diagnostic features, are used to tific study. The pedon is this unit. A soil pedon is classify soils in Soil Taxonomy. the smallest volume that can be called a soil, and Soil moisture regimes express the changes in it is roughly polygonal in shape. The lower bound- soil water with time. Aquic soil moisture regimes ary is the somewhat vague boundary between soil are characterized by water-saturation. Aridic, and nonsoil at the approximate depth of root pen- ustic, and udic are three regimes with an increas- etration. Lateral dimensions are large enough to ing amount of water available for plant growth represent any horizon. The area of a pedon ranges during the summer. Soils with perudic regime are from I to 10 square meters. The pedon is to the continuously wet. Xeric soil moisture regime is soil body what an oak tree is to an oak forest. A characterized by cool, rainy winters and hot, dry soil body is composed of many pedons, which is summers. why a soil body is called a polypedon (see Figure Soil temperature regimes reflect the mean aver- 17.6). age soil temperature (MAST) in the root zone. There are more than 16,000 soils (series) in the United States and they are classified into orders, suborders, great groups, subgroups, families, and series based on the presence of various diagnos-

FIGURE 17.6 Illustration of the relationship tic horizons and other criteria. between the pedon and polypedon. The pedon is the smallest volume that can be called a soil.

REFERENCES Boul, S. W., F. D. Hole, and R. J. McCracken. 1980. Soil Genesis and Classification. 2nd ed. Iowa State Uni- versity Press, Ames. Fanning, D. S., and M. C. B. Fanning. 1989. Soil Mor- phology, Genesis, and Classification. Wiley, New York. Foth, H. D. and J. W. Schafer. 1980. Soil Geography and Land Use. Wiley, New York. Soil Survey Staff. 1975. Soil Taxonomy. USDA Agr. Handbook 436, Washington, D.C. Soil Survey Staff. 1987. "Keys to Soil Taxonomy." Soil Management Support Services Tech. Monograph 6. USDA Washington, D.C. Soil Survey Staff, 1989. Amendments to Soil Taxonomy. USDA, Washington, D.C. Wilding, L. P., N. E. Smeck, and G. F. Hall. 1983. Pedo- genesis and Soil Taxonomy: I Concepts and Interac- tions. Elsevier, New York CHAPTER 18

SOIL GEOGRAPHY AND LAND USE

The extent and abundance of the orders for the meters below the upper boundary of the argillic United States and the world are shown in Table horizon or 1.8 meters below the soil surface, 18.1. Aridisols are the most abundant on a world- whichever is greater. wide basis, occupying 19.2 percent of the land, In humid forested regions, many of the Alfisols whereas Mollisols are the most abundant soils in have an albic (E) horizon between the ochric and United States with 24.6 percent. Inceptisols and argillic horizons. Alfisols that develop in regions Alfisols are ranked 2 and 3, respectively, in both or areas adjacent to grasslands lack E horizons the United States and the world. Histosols and and have properties very similar to Mollisols. The Oxisols are the least abundant soils in United limited accumulation of organic matter and ab- States, and Histosols and Andisols are the least sence of a mollic horizon may result in a grass- abundant soils worldwide. land soil being an Alfisol rather than a Mollisol. The geographic distribution of soils in the This is the situation in many areas in which the United States is given in Figure 18.1 and for the dominant soils have an ustic moisture regime. For world in Figure 18.2. The consideration of the these reasons, many Alfisols are quite similar to soils is primarily at the suborder level. Mollisols in native fertility and ability to produce crops. Large areas of each of the suborders of Alfisols occur in United States. ALFISOLS The central concept of Alfisols is soils with an Aqualfs ochric epipedon and an argillic subsurface hori- Aqualfs have an aquic soil moisture regime and zon, only moderately weathered and leached, and are shown in Figure 18.1 as Ala. In the area west enough available water for the production of of Lake Erie in Ohio and Michigan, the Aqualfs crops at least three months during the year. There have a naturally high water table. The Aqualfs is sufficient exchangeable calcium, magnesium, formed mainly on glacial lake beds from fine- potassium, and sodium to saturate 35 percent or textured parent material. They occur on nearly 0 more of the CEC (at pH 8.3) at a depth of 1.25 percent slopes and have high natural fertility.

285 286 SOIL GEOGRAPHY AND LAND USE

When artificially drained, the soils are very pro- ter for collection in reservoirs, and they are used ductive for corn, soybeans, wheat, and sugar for some grazing of cattle. The Boralfs, area Ala in beets. The soils are poorly suited to septic filter- Wisconsin and Minnesota, developed from recent field sewage disposal systems and other uses glacial parent materials. Much of the land is found where natural soil wetness is undesirable. in forests. Areas in agriculture are devoted to Another area of Aqualfs (Ala in Figure 18.1) dairying and specialty crops, such as potatoes. A occurs in southern Illinois and west into Missouri. very large area of Boralfs, similar to that in Minne- These soils are wet because of thick argillic hori- sota and Wisconsin, occurs in the Soviet Union zons (claypans). The soils developed on nearly (area Ala of Figure 18.2). Moscow is located in level upland areas on land surfaces that were this area; important crops are rye, oats, flax, sugar formed by some of the earlier glaciations. Another beets, and forage crops. Dairy cattle and poultry, area of claypan Aqualfs occurs in southwestern together with pigs, are important livestock prod- Louisiana, where wetland rice is a major crop. ucts. Boralfs represent some of the most northerly soils used for agriculture.

B Boralfs are the northern or cool Alfisols with cryic U or frigid temperature regime. The mean annual Udalfs are warmer than Boralfs and have a udic soil temperature is warmer than 0° C (32° F) and soil moisture regime. The major area of Udalfs in colder than 8° C (47° F). Forestry rather than agri- United States occurs east of the Prairie region in culture tends to be the major land use. The soils the north-central states. These areas (A3a in Fig- are too cold for winter wheat, corn, and soybeans. ure 18.1) have trees for native vegetation because Two kinds of Boralf areas are shown in Figure they are more humid than the major grasslands 18.1 -A2a and A2s. The A2s areas occur on steep located to the west. The Udalfs have developed slopes in the Rocky Mountains. The soils of the mainly from mid- to late-Pleistocene loess and till. Black Hills in western South Dakota are also In Indiana, Ohio, and southern Michigan, the soils mainly Boralfs. These areas are important for rec- are nearly as productive as are the grassland soils reation and forestry uses, they provide runoff wa- (Mollisols) in Illinois and Iowa. These soils are Legend for general soil map of the United States. 1 1 S1-Cryands with Cryochrepts, Cryumbrepts, and Cryorthods; steep. ALFISOLS Udands 1 1 Aqualfs 52-Udands and Ustands, Ustolls, and Tropofolists; Ala-Aqualfs with Udalfs, Haplaquepts, Udolls; gently sloping. moderately sloping to steep. Boralfs Ala-Boralfs with Udipsamments and Histosols; gently and ARIDISOLS moderately sloping. Argids A2S-Cryoboralfs with Borolls, Cryochrepts, Cryorthods, and D1a-Argids with Orthids, Orthents, Psamments, and Ustolls; rock outcrops; steep. gently and moderately sloping Udalfs D1S-Argids with Orthids, gently sloping; and Torriorthents, A3a-Udalfs with Aqualfs, Aquolls, Rendolls, Udolls, and gently sloping to steep. Udults; gently or moderately sloping. Orthids Ustalfs D2a-Orthids with Argids, Orthents, and Xerolls; gently or A4a-Ustalfs with Ustochrepts, Ustolls, Usterts, moderately sloping. Ustipsamments, and Ustorthents; gently or moderately D2S-Orthids, gently sloping to steep, with Argids, gently sloping. sloping; lithic subgroups of Torriorthents and Xerorthents, both steep. Xeralfs A5S1-Xeralfs with Xerolls, Xerothents, and Xererts; moderately sloping to steep. ENTISOLS A5S2-Ultic and lithic subgroups of Haploxeralfs with Aquents Andisols, Xerults, Xerolls, and Xerochrepts; steep. Ela-Aquents with Quartzipsamments, Aquepts, Aquolls, and Aquods; gently sloping. ANDISOLS Orthents Cryands E2a-Torriorthents, steep, with borollic subgroups of 11 a-Cryands with Cryaquepts, Histosols, and rock land; gently Aridisols; Usterts and aridic and vertic subgroups of Borolls; and moderately sloping. gently or moderately sloping. 288 SOIL GEOGRAPHY AND LAND USE

E2b-Torriorthents with Torrerts; gently or moderately Psamments, Ustorthents, Aquepts, and Albolls; gently or sloping. moderately sloping. E2c-Xerothents with Xeralfs, Orthids, and Argids; gently Ustolls sloping. M4a-Udic subgroups of Ustolls with Orthents, Ustochrepts, E2S1-Torriorthents; steep, and Argids, Torrifluvents, Ustolls, Usterts, Aquents, Fluvents, and Udolls; gently or moderately and Borolls; gently sloping. sloping. E2S2-Xerorthents with Xeralfs and Xerolls; sleep. M4b-Typic subgroups of Ustolls with Ustalfs, Ustipsamments, E2S3-Cryorthents with Cryopsamments and Cryands; gently Ustorthents, Ustochrepts, Aquolls, and Usterts; gently or sloping to steep. moderately sloping. Pasmments M4c-Aridic subgroups of Ustolls with Ustalfs, Orthids, E3a-Quartzipsamments with Aquults and Udults; gently or Ustipsamments, Ustorthents, Ustochrepts, Torriorthents, moderately sloping. Borolls, Ustolls, and Usterts, gently or moderately sloping. E3b-Udipsamments with Aquolls and Udalfs; gently or M4S-Ustolls with Argids and Torriorthents; moderately moderately sloping. sloping or steep. E3c-Ustipsamments with Ustalfs and Aquolls; gently or Xerolls moderately sloping. M5a-Xerolls with Argids, Orthids, Fluvents, Cryoboralfs, HISTOSOLS Cryoborolls, and Xerorthents; gently or moderately sloping. M5S-Xerolls with Cryoboralfs, Xeralfs, Xerothents, and Histosols Xererts; moderately sloping or steep. H1a-Hemists with Psammaquents and Udipsamments; gently sloping. SPODOSOLS H2a-Hemists and Saprists with Fluvaquents and Aquods Haplaquepts; gently sloping. S1a-Aquods with Psammaquents, Aquolls, Humods, and H3a-Fibrists, Hemists, and Saprists with Psammaquents; Aquults; gently sloping gently sloping. Orthods I NCEPTISOLS S2a-Orthods with Boralfs, Aquents, Orthents, Psamments, Aquepts Histosols, Aquepts, Fragiochrepts, and Dystrochrepts; gently 12a-Haplaquepts with Aqualfs, Aquolls, Udalfs, and or moderately sloping. Fluvaquents; gently sloping. S2S1-Orthods with Histosols, Aquents, and Aquepts; 1 2P-Cryaquepts with cryic great groups of Orthents, moderately sloping or steep. Histosols, and Ochrepts; gently sloping to steep. S2S2-Cryorthods with Histosols; moderately sloping or steep. S2S3-Cryorthods with Histosols, Udands and Aquepts; gently Ochrepts sloping to steep. 1 3a-Cryochrepts with cryic great groups of Aquepts, Histosols, and Orthods; gently or moderately sloping. ULTISOLS 1 3b-Eutrochrepts with Uderts; gently sloping. Aquults 1 3c-Fragiochrepts with Fragiaquepts, gently or moderately Ula-Aquults with Aquents, Histosols, Quartzipsamments, and sloping; and Dystrochrepts, steep. Udults; gently sloping. 13d-Dystrochrepts with Udipsamments and Haplorthods; gently sloping. Humults 13S-Dystrochrepts, steep, with Udalfs and Udults; gently or U2S-Humults with Andisols, Tropepts, Xerolls, Ustolls, Udox, moderately sloping. Torrox, and rock land; gently sloping to steep. Udults U3a-Udults with Udalfs, Fluvents, Aquents, Umbrepts Quartzipsamments, Aquepts, Dystrochrepts, and Aquults; 14a-Haplumbrepts with Aquepts and Orthods; gently or gently or moderately sloping. moderately sloping. U3S-Udults with Dystrochrepts; moderately sloping or steep. 1 4S-Haplumbrepts and Orthods; steep, with Xerolls and Andisols; gently sloping. VERTISOLS MOLLISOLS Uderts Via-Uderts with Aqualfs, Eutrochrepts, Aquolls, and Ustolls; Aquolls gently sloping. Mia-Aquolls with Udalfs, Fluvents, Udipsamments, Ustipsamments, Aquepts, Eutrochrepts, and Borolls; gently Usterts sloping. V2a-Usterts with Aqualfs, Orthids, Udifluvents, Aquolls, Ustolls, and Torrerts; gently sloping. Borolls M2a-Udic subgroups of Borolls with Aquolls and Ustorthents; gently sloping. M2b-Typic subgroups of Borolls with Ustipsamments, Areas With Little Soil Ustorthents, and Boralfs; gently sloping. X1-Salt flats. M2c-Aridic subgroups of Borolls with Borollic subgroups of X2-Rock land (plus permanent snow fields and glaciers). Argids and Orthids, and Torriorthents; gently sloping. M2S-Borolls with Boralfs, Argids, Torriorthents, and Ustolls; moderately sloping or steep. Slope Classes Udolls Gently sloping-Slopes mainly less than 10 percent, including M3a-Udolls, with Aquolls, Udalfs, Aqualfs, Fluvents, nearly level. FIGURE 18.2 Broad schematic map of the soil orders and suborders of the world.

ALFISOLS 289

Moderately sloping-Slopes mainly between 10 and 25 percent. . Steep-Slopes mainly steeper than 25 percent. A1 B-. A H, SOILS OF THE WORLD Distribution of Orders and Principal A S, Suborders and Great Groups A2 U- , . A ALFISOLS-S A2 A A A ; 90 A2 H

FIGURE 18.2 Broad schematic map of the soil orders and suborders of the world.

U. S. DEPARTMENT OF AGRICULTURE FIGURE 18.2 (Continued)

290 SOIL GEOGRAPHY AND LAND USE

A2d with Ochrepts A3f with Ustolls A2e with Troporthents A3g with Ustorthents A2f with Udorthents A3h with Ustox A3j Plinthustalfs with Ustorthents A3 Ustalfs-temperate to hot, dry more than 90 cumulative days during periods when temperature is suitable for plant growth. A4 Xeralfs-temperate or warm, moist in Ma with Tropepts winter and dry more than 45 consecutive A3b with Troporthents days in summer. A3c with Tropustults Ma with Xerochrepts A3d with Usterts A4b with Xerorthents Me with Ustochrepts A4c with Xerults

ALFISOLS 291

ARIDISOLS-Soils with pedogenic horizons, 1 2 Aquepts-seasonally wet. usually dry in all horizons and never moist as 1 2a Cryaquepts with Orthents l ong as 90 consecutive days during a period l 2b Haplaquepts with Salorthids when temperature is suitable for plant growth. 1 2c Haplaquepts with Humaquepts 1 2d Haplaquepts with Ochraqualfs Dl Aridisols-undifferentiated. 1 2e Humaquepts with Psamments D1 a with Orthents 1 2f Tropaquepts with Hydraquents D1b with Psamments 1 2g Tropaquepts with Plinthaquults D1c with Ustalfs 1 2h Tropaquepts with Tropaquents D2 Argids-with horizons of clay 1 2j Tropaquepts with Tropudults accumulation. 1 3 Ochrepts-thin, light-colored surface D2a with Fluvents horizons and little organic matter. D2b with Torriorthents 1 3a Dystrochrepts with Fragiochrepts ENTISOLS-Soils without pedogenic horizons; 1 3b Dystrochrepts with Udox either usually wet, usually moist, or usually dry. 1 3c Xerochrepts with Xerolls El Aquents-seasonally or perennially wet. 1 4 Tropepts-continuously warm or hot. E1a Haplaquents with Udifluvents 1 4a with Ustalfs E1b Psammaquents with Haplaquents 1 4b with Tropudults Etc Tropaquents with Hydraquents 1 4c with Ustox E2 Orthents-loamy or clayey textures, many 1 5 Umbrepts-dark-colored surface horizons shallow to rock. with medium to low base supply. E2a Cryothents 1 5a with Aqualfs E2b Cryorthents with Orthods E2c Torriorthents with Aridisols MOLLISOLS-Soils with nearly black, organic- E2d Torriorthents with Ustalfs rich surface horizons and high base supply; E2e Xerorthents with Xeralfs either usually moist or usually dry. E3 Psamments-sand or loamy sand textures. Ml Albolls-light gray subsurface horizon over E3a with Aridisols slowly permeable horizon; seasonally wet. E3b with Udox M1a with Aquepts E3c with Torriorthents M2 Borolls-cold. E3d with Ustalfs M2a with Aquolls E3e with Ustox M2b with Orthids E3f shifting sands M2c with Torriorthents E3g Ustipsamments with Ustolls M3 Rendolls-subsurface horizons have much calcium carbonate but no accumulation of HISTOSOLS-Organic soils. clay. Hl Histosols-undifferentiated. M3a with Usterts Hl a with Aquods M4 Udolls-temperate or warm, usually moist. H1b with Boralfs M4a with Aquolls H1c with Cryaquepts M4b with Eutrochrepts M4c with Humaquepts I I NCEPTISOLS-Soils with pedogenic horizons of alteration or concentration but without M5 Ustolls-temperate to hot, dry more than accumulations of translocated materials other 90 cumulative days in year. than carbonates or silica; usually moist or moist M5a with Argialbolls for 90 consecutive days during a period when M5b with Ustalfs temperature is suitable for plant growth. M5c with Usterts M5d with Ustochrepts 11 Andepts-amorphous clay or vitric volcanic ash or pumice. The suborder M6 Xerolls-cool to warm, moist in winter and Andepts was converted to the Andisol dry more than 45 consecutive days in order in 1989. summer. I1a Dystrandepts with Ochrepts M6a with Xerorthents

292 SOIL GEOGRAPHY AND LAND USE

OXISOLS-Soils with pedogenic horizons that U3j Tropudults with Tropepts are mixtures principally of kaolin, hydrated U3k Tropudults with Tropudalfs oxides, and quartz, and are low in weatherable U4 Ustults-warm or hot; dry more than 90 minerals. cumulative days in the year. 01 Udox-hot, nearly always moist. U4a with Ustochrepts O1a with Plinthaquults U4b Plinthustults with Ustorthents 01 b with Tropudults U4c Rhodustults with Ustalfs 02 Ustox-warm or hot, dry for long periods U4d Tropustults with Tropaquepts but moist more than 90 consecutive days U4e Tropustults with Ustalfs i n the year. V VERTISOLS-Soils with high content of swelling 02a with Plinthaquults clays; deep, wide cracks develop during dry 02b with Tropustults periods. 02c with Ustalfs V1 Uderts-usually moist in some part in most SPODOSOLS-Soils with accumulation of years; cracks open less than 90 cumulative amorphous materials in subsurface horizons; days in the year. usually moist or wet. Via with Usterts S1 Spodosols-undifferentiated. V2 Usterts-cracks open more than 90 Sta cryic temperature regimes; with cumulative days in the year. Boralfs V2a with Tropaquepts Sib cryic temperature regimes; with V2b with Tropofluvents Histosols V2c with Ustalfs S2 Aquods-seasonally wet. X Soils in areas with mountains-Soils with various S2a Haplaquods with Quartzipsamments moisture and temperature regimes; many steep S3 Humods-with accumulations of organic slopes; relief and total elevation vary greatly from matter in subsurface horizons. place to place. Soils vary greatly within short S3a with Hapludalfs distances and with changes in altitude; vertical zonation common. S4 Orthods-with accumulations of organic matter, iron, and aluminum in subsurface X1 Cryic great groups of Entisols, Inceptisols, horizons. and Spodosols. S4a Haplorthods with Boralfs X2 Boralfs and cryic great groups of Entisols and Inceptisols. ULTISOLS-Soils with subsurface horizons of clay accumulation and low base supply; usually X3 Udic great groups of Alfisols, Entisols, and moist or moist for 90 consecutive days during a Ultisols; Inceptisols. period when temperature is suitable for plant X4 Ustic great groups of Alfisols, Inceptisols, growth. Mollisols, and Ultisols. U1 Aquults-seasonally wet. X5 Xeric great groups of Alfisols, Entisols, U1a Ochraquults with Udults I nceptisols, Mollisols, and Ultisols. U1b Plinthaquults with Udox X6 Torric great groups of Entisols; Aridisols. U1c Plinthaquults with Plinthaquox X7 Ustic and cryic great groups of Alfisols, U1d Plinthaquults with Tropaquepts Entisols, Inceptisols, and Mollisols; ustic U2 Humults-temperate or warm and moist all great groups of Ultisols; cryic great groups of year; high content of organic matter. of Spodosols. U2a with Umbrepts X8 Aridisols, torric and cryic great groups of U3 Udults-temperate to hot; never dry more Entisols, and cryic great groups of than 90 cumulative days in the year. Spodosols and Inceptisols. U3a with Andepts (Udands) Z MISCELLANEOUS U3b with Dystrochrepts Zi I cefields. U3c with Udalfs Z2 Rugged mountains-mostly devoid of soil U3d Hapludults with Dystrochrepts (includes glaciers, permanent snow fields, We Rhodudults with Udalfs and, in some places, areas of soil). U3f Tropudults with Aquults U3g Tropudults with Hydraquents . . . Southern limit of continuous permafrost. U3h Tropudults with Udox - -Southern limit of discontinuous permafrost. ALFISOLS 293

vanna vegetation. Much of the land is devoted to cattle grazing, and warm temperatures allow cotton to be grown. The largest area in the world in which Alfisols are the dominant soils occurs south of the Sahara Desert in Africa. The soils are Ustalfs (see area A3a in Figure 18.2). The soils have undergone a long period of weathering; many of them have kandic horizons, and would normally be Ultisols. However, calcareous dust from the Sahara Desert is carried south by seasonal winds and is depos- FIGURE 18.3 Many Udalfs occur on gentle slopes of glaciated areas in the north-central part of the United ited on the soils: This has kept the base saturation States and New York, where a wide variety of crops is at 35 percent or more. Considerable cattle raising, grown and dairying is a major industry. subsistence farming, and shifting cultivation occur in this region. The large size of raindrops of the tropical thunderstorms contributes to high well suited to the production of a wide variety of rates of water runoff and soil erosion on exposed crops, especially corn, soybeans, wheat, and for- land. ages (see Figure 18.3). Udalfs have developed along the lower Mississippi River in loess and are Xeralfs similar to Udalfs that developed from loess in the Xeralfs have a zeric moisture regime and are the Midwest. A Udalf is shown in Color Plate 5. dominant soils in the coastal ranges and inland Many of the best agricultural soils in northwes- mountains in California and southern Oregon tern Europe are Udalfs comparable to those of the (see areas A5s1 and A5s2 in Figure 18.1). Natural north-central United States in that they developed in glacial parent materials of similar age. In both areas, dairying is a major industry. FIGURE 18.4 Grazing of cattle is important on Soils of the Blue Grass region in central Ken- Xeralfs on the lower slopes of the Sierra Nevada Mountains in California. On the cooler and more tucky are predominantly Udalfs that developed humid upper slopes are Xerults, Ultisols with a xeric from a high phosphorus limestone capped by a moisture regime. thin layer of loess. Early settlers prized the soils for growing food and, today, they are used for raising racehorses in addition to being used for general agriculture.

Ustalfs Ustalfs have an ustic moisture regime and tend to occur where temperatures are warm or hot and the vegetation is grassland or savanna. The large areas dominated by Ustalfs in Texas and New Mexico (A4a in Figure 18.1) occur south of areas where the soils are mainly Mollisols. In these Ustalf areas, higher temperature and sandy parent materials have contributed to the development of soils with ochric epipedons under grass or sa- 294 SOIL GEOGRAPHY AND LAND USE vegetation is a mixture of annual grasses, forbs, Andisols with melanic epipedons, are visible from and shrubs. Many of these sloping lands are used the air in the Andes of South America. for animal grazing, and some areas are irrigated for the production of horticultural crops. In the mountains at highest elevations with greater pre- Suborders cipitation, Ultisols occur as shown in Figure 18.4. Most Andisols suborders are based on moisture and temperature regimes. Aquands have an aquic moisture regime, Cryands are cold (cryic temperature regime or colder), Torrands have an aridic ANDISOLS moisture regime, and Xerands have a xeric mois- Andisols are weakly developed soils produced by ture regime. Vitrands have the greatest content of the weathering of volcanic ejecta. Although An- glassy or vitreous materials. Sufficient weathering disols occupy less than 1 percent of the world's of Vitrands and disappearance of glass results in land surface, they are important soils of the Pa- Udands with udic moisture regime or Ustands cific Ring of Fire-that ring of active volcanoes with ustic moisture regime. along the western coast of the American con- The wide range in temperature and moisture tinents, across the Aleutian Islands, down Japan, regimes is associated with great variation in na- the Philippine Islands, other Pacific Islands, and tive vegetation from bamboo, grass, sedges, and down to New Zealand. The most important areas forest. Xerands are the dominant Andisols of the of Andisols in the United States occur in the Pa- Pacific northwest in the United States. Udands cific northwest and Hawaiian Islands. and Ustands are the dominant Andisols of the Island of Hawaii where they are important soils for sugarcane production and cattle grazing. Genesis and Properties Locations where soils are likely to be Andisols An important feature of Andisols is the presence are shown on the world soil map as mountainous of volcanic glass. The dominant process in most areas, X areas. One area of volcanic ash soils is Andisols is one of weathering and mineral translocated in northern New Zealand. The soils are formation. Volcanic glass weathers rapidly pro- Andepts in the legend of Figure 18.2. In 1989 the ducing allophane and oxides of aluminum and Andept suborder was converted to the Andisol iron. Translocation and accumulation within the order. soil are minims). Repeated ash falls maintain the weakly developed nature of the soils and buried horizons are common. ARIDISOLS Andisols have andic soil properties. The weathered minerals, including allophane and oxides of Aridisols have an aridic moisture regime and are aluminum and iron, are amorphous or short- the dominant soils of deserts. Large areas of des- range minerals with enormous surface area and ert occur in northern Africa (Sahara Desert), cen- great capacity to complex with organic matter. tral Eurasia, central and western Australia, in Andisols are characterized by high organic matter southern South America east of the Andes Moun- content, ion exchange capacity, and low bulk tains, in northern Mexico, and in the southwest- density. The capacity to hold available water for ern United States. Small deserts exist on many plant growth is high. Soils are generally con- islands in the rain shadow of mountains, as in the sidered fertile, although high fixation and low Hawaiian and Caribbean islands. Aridisols are the availability of phosphorus are sometimes a prob- most abundant soils worldwide and the fifth most lem. Many Andisols have melanic epipedons. abundant soils in the United States (see Table Large areas of intensively cropped black soils, 18.1). ARIDISOLS 295

lation and limited weathering, leaching, and eluviation. A striking feature of most Aridisols is a zone below the surface in which calcium carbonate has accumulated to form a calcic (k) hori- zone as a result of limited leaching by water. Winds play a role by removing the finer particles from exposed areas and, where gravel occurs in the soil, a surface layer of gravel (desert pavement) is formed (see Figure 18.5). Some Aridisols, however, have a well- developed argillic horizon (Bt), which indicates considerable weathering and clay movement. The occurrence of argillic horizons suggests that a more humid climate existed many years ago. The Mohave is a common Aridisol in the Sonoran Des- ert of Arizona; and some data are given in Table 18.2 to illustrate properties commmonly found in Aridisols. Note the Bt horizon (argillic) at the 10- to 69- centimeter depth. This is underlain by Bk and Ck horizons that contain 10 and 22 percent calcium carbonate, respectively. The horizons have low organic matter contents and low carbon- nitrogen ratios. The argillic horizon has high CEC because of clay accumulation. Significant exchangeable sodium and high pH values reflect FIGURE 18.5 Vegetation on the Sonoran Desert in Arizona. Note gravel pavement in the foregound limited leaching, at least, in recent years. A photo- because of removal of fine particles by the wind. graph of an Aridisol with A, Bt, and Ck horizons is shown in Color Plate 6. Shrubs, the dominant vegetation, give way to bunch grasses with increasing precipitation. A S Plants are widely spaced and use the limited wa- Aridisols are placed in suborders on the basis of ter supply quite effectively. Many desert plants the presence or absence of an argillic horizon. grow and function during the wetter seasons and Orthids do not have argillic horizons, whereas go dormant during the driest seasons. Suprisingly, Argids do. As shown in Table 18.2, the Mohave there can be great plant diversity and a consider- soil has an argillic horizon and is an Argid. The able amount of vegetation, as seen in Figure 18.5. general distribution of Orthids and Argids in the On the more humid or eastern edge of the arid United States is shown in Figure 18.1 and for the region in southwestern United States, the desert world in Figure 18.2. bunch grasses give way to taller and more vigor- It is believed that Orthids are young Aridisols ous grasses; Aridisols merge with Alfisols and and that Argids, by comparison, are much older Mollisols. Aridisols. There is evidence that the Orthids in the United States developed largely within the past G P 25,000 years in an arid climate. Orthids are usu- In arid regions, there is a slow rate of plant ally located where recent alluvium has been de- growth, resulting in low organic matter accumu- posited. Argids are common on old land surfaces 296 SOIL GEOGRAPHY AND LAND USE

in any landscape where there has been enough grazing of cattle and sheep and production of time for genesis of an argillic horizon during a crops by irrigation are the two major agricultural humid period that existed more than 25,000 years land uses. The use of land for grazing is closely ago. About 75 percent of the Aridisols in United related to precipitation, which largely determines States are Argids. Entisols occur on the most re- the amount of forage produced. Some areas are cent land surfaces. The desert landscape shown too dry for grazing, whereas other areas receive in Figure 18.6 has land surfaces with greatly differ- more favorable precipitation and thus may be ent ages. used for summer grazing on mountain meadows. As many as 35 hectares (75 acres) or more in the drier areas are required per head of cattle, thus L U A making large farms or ranches a necessity. Most Aridisols of the western United States occur al- ranchers supplement range forage by producing most entirely within a region called the "western some crops on a small irrigated acreage as shown range and irrigated region." As the name implies, in the left foreground of Figure 18.6. Overgrazing,

FIGURE 18.6 Grazing lands dominated by Aridisols. Soils in the area are closely related to age of land surface with Aridisols on the older surfaces and Entisols on the youngest surfaces. Note the small irrigation reservoir and irrigated cropland near ranch headquarters. (USDA photograph by B. W. Muir.) ENTISOLS 297 which results in the invasion of less desirable plant species and increased soil erosion, is the major hazard in the use of these grazing lands. In Arizona, for example, 1 or 2 percent of the land is irrigated. Most of the irrigated land is located on soils developed from alluvial parent materials along streams and rivers. Here, the land is nearly level, irrigation water is obtained from the river, and water is distributed over the field by gravity. The alkaline nature of Aridisols may cause deficiencies of various micronutrients on certain crops. Major crops include alfalfa, cotton, citrus fruits, vegetables, and grain crops. In Arizona, crop production on only I or 2 percent of the land that is irrigated accounts for 60 percent of the total farm income. Grazing, by contrast, uses 80 percent of the land and accounts for only 40 percent of the total farm income.

ENTISOLS Entisols are soils of recent origin. The central concept is soils developed in unconsolidated earth, or parent material, with no genetic horizons except an A horizon. Some Entisols have rock FIGURE 18.7 An Entisol developing in parent close to the surface and have A and R horizons as material formed by the weathering of granite in the shown in Figure 18.7. All soils that do not fit into U.S. Rocky Mountains. one of the other 10 orders are Entisols. The result is that the Entisol order is characterized by great diversity. The largest area in which soils are mainly Aquents worldwide occurs on the North China Plain near the mouths of Yellow, Yangtze, and Aquents several other rivers (area E1a in Figure 18.2). It is a Small areas of Aquents are widely distributed. In vast floodplain where inhabitants must cope with the United States, however, the only major area occasional catastrophic floods. This large flood- shown in Figure 18.1 is in southern Florida (area plain is one of the most densely populated and E1a). Parent materials are mainly marine sands intensively cropped areas of the world. and marl. The soils are wet as a result of a naturally high water table. Although they are drained for cropping, water for irrigation is pumped from a Fluvents shallow water table when needed. The frost-free Fluvents develop in alluvium of recent origin. weather in southern Florida makes these soils They are, in large part, soils having properties that valuable for production of winter vegetables and have been inherited from the parent material. For fruits. a soil to be a Fluvent it must have, with increasing 298 SOIL GEOGRAPHY AND LAND USE depth, an irregular distribution of organic matter that reflects the organic matter content of different layers of recently deposited alluvium. The organic matter in soils formed from alluvium decomposes over time. Therefore, in the absence of continued flooding and deposition of alluvium, a normal gradual decrease in organic matter content with increasing soil depth is produced with time. In this way, a Fluvent may develop in the absence of continued flooding and deposition of alluvium into an Inceptisol, an Aquept. For this reason many soils on large river valley floodplains, such FIGURE 18.8 Cow-calf production is the major land as the Mississippi River floodplains, are not use on the Psamments of the Nebraska Sandhills. Fluvents. Maintenance of a grass cover is essential to prevent Alluvial sediments were deposited in the Impe- serious wind erosion. rial Valley in southeastern California by several years of uncontrolled flooding of the Colorado maintain a vegetative cover to control wind River when the valley underwent widespread de- erosion. velopment of irrigation. Some Fluvents have de- Psamments occupy most of the central ridge of veloped in these recent sediments, however, most the Florida peninsula. The hyperthermic tempera- of the soils are Orthents. The uncontrolled flood- ture regime provides frost-free winters and makes ing created the Salton Sea in a depression below the soils prized for citrus production. Outside the sea level north of the valley. Now, the Salton Sea United States many large areas in north and south serves as a sink for the salt in the drainage water Africa, Australia, and Saudi Arabia are dominated from the irrigated fields. by Psamments.

P O Psamments develop in sand parent materials and The suborder Orthents includes all Entisols that do not have an aquic moisture regime. do not fit into one of the other suborders. As a Psamments develop when unstablized sand result, Orthents are very diverse and form in par- dunes become stablized by vegetation. The tex- ent materials finer in texture than sands and with- ture must be loamy fine-sand or coarser to a depth out an aquic moisture regime. Delayed genesis is of 1 meter or more, or to a hard rock layer. The caused by active erosion on steep slopes, short Sandhills of north-central Nebraska (see area E3c period of time, and/or hardness of rock close to of Figure 18.1) are dominated by Psamments with the soil surface. Where the parent materials are an ustic moisture regime. These soils are Ustip- derived from unconsolidated rock, A-C soils oc- samments. This area consists of sand dunes sta- cur. A-R soils occur where rock is near the soil bilized by mid- and tall grasses and used for cattle surface (see Figure 18.7). Active erosion on steep grazing. The major product is feeder stock that is slopes accounts for many Orthents in the Bad- sold for fattening in feedlots. A typical Sandhills lands of the western United States, and active landscape is shown in Figure 18.8. Numerous erosion, together with rock close to the soil sur- small lakes in the area provide watering places. face, account for many of the Orthents in the The major soil management challenge is to con- Rocky Mountains. Orthents are common in the trol grazing so that productive forage species valleys of California, including the large Central HISTOSOLS 299

Valley (area Etc of Figure 18.1). The soils have a diate clay contents have intermediate amounts of xeric moisture regime, are mainly Xerorthents, organic carbon (see Figure 9.5). and were formed in sediments carried to the val- Many Histosols have developed in organic ma- ley floor as a result of erosion of the surrounding terials of great depth (many meters deep). Others hills and mountains. The soils are intensively irri- have developed in parent materials containing gated, and many high value crops are produced. organic layers of varying thickness. Where the As a result, the value of agricultural crops pro- organic material overlies rock at a shallow depth, duced in California is the largest of any state (see Histosols can be thin. Figure 18.9).

Histosol Suborders During water saturation, organic matter resists HISTOSOLS decomposition but decomposes when the soil drains and desaturates. The degree of decom- Histosols are organic soils, and they have been position of the organic fibers or plant materials called bogs, peats, mucks, and moors. Most His- increases with increasing time of drainage or des- tosols are water saturated for most of the year aturation. Three degrees of decomposition are unless they have been drained. Histosols contain recognized and reflected in the suborders Fibrists, organic soil materials to a significant depth. Or- Hemists, and Saprists. Fibrists consist largely of ganic soil material contains a variable amount of plant remains so little decomposed that their bo- organic carbon, depending on the clay content. tanical origin can be determined readily. Hemists Soil material containing 60 or more percent clay are decomposed sufficiently so that the botanical has an organic carbon content of 18 or more per- origin of as much as two-thirds of the materials cent. Material containing no clay has an organic cannot be determined. Saprists consist of almost carbon content of 12 or more percent. Interme- completely decomposed plant remains and they usually have a black color. Histosol horizons are 0 horizons: Oi, Oe, and Oa refer to 0 horizons with fabric, hemic, and FIGURE 18.9 Orthents are the most important sapric organic materials, respectively. Folists are agricultural soils in California in large, broad, and the other suborder and consist of organic soil nearly level valleys. A wide variety of crops is grown, materials resulting from the deposition of leaf or including fruits, nuts, vegetables, and farm crops. Cotton is shown in the foreground and large stacks of foliar material. Folists may be thin over hard rock, baled alfalfa in the rear. and it is likely that they may never be water saturated. One eighth of the soils in Michigan are Histo- sols, which occur in all the counties, with only a few areas being quite extensive. In Figure 18.1, two large areas of Hemists (H1a) are shown in northern Minnesota, and one area is shown in the lower Mississippi River delta in Louisiana. The Everglades in Florida contain Fibrists, Hemists, and Saprists (see area H3a in Figure 18.1). An Everglades landscape is shown in Figure 18.10. Large acreages of Histosols occur in northern Canada (see H I b and H1c in Figure 18.2). 300 SOIL GEOGRAPHY AND LAND USE

FIGURE 18.10 Histosol landscape of the Florida Everglades.

L U H A major limitation of northern Histosols for agri- FIGURE 18.11 In 1924 the bottom of this 9-foot cultural use is frost hazard. The Histosols tend to concrete post was placed on the underlying limestone at Belle Glade, Florida, and the top of the occupy the lowest position in the landscape and post was at ground level. This photograph, which are the first areas to freeze. Use for crop produc- was taken in 1972, shows that subsidence of the tion requires water drainage, which enhances de- organic soil (Histosol) has been about 1 inch per composition of the organic matter. The sub- year. sidence and loss of soil by oxidation and disappearance of the soil is a major concern, as shown in Figure 18.11. A large acreage in the typically show little evidence of eluviation, illuvia- Florida Everglades is used primarly for the pro- tion, or extreme weathering. They are too old to be duction of sugarcane. During 50 years of agricul- Entisols and lack sufficient diagnostic features to ture the soils subsided an amount equal to an be placed in one of the other orders. Inceptisols amount of soil that required 1,200 years to form. occur in all climatic zones where there is some Since the Histosols of the Everglades overlie lime- leaching in most years, and are the second most stone, many areas now cultivated will be too thin abundant soils both in the United States and for continued cultivation past the year 2000. The worldwide. In the United States about 70 percent likely use for such soils will then be pasture, of the Inceptisols are Aquepts, 26 percent are which requires no tillage. Ochrepts, and 4 percent are Umbrepts. Other problems in the use of Histosols are fire hazards and wind erosion during dry seasons. The soils contain few minerals for supplying many of the plant nutrients; however, the organic A matter contributes to high nitrogen-supplying Aquepts are wet Inceptisols that have an aquic power. Despite the problems in their use, many moisture regime, unless they are artificially Histosols are intensively managed for the produc- drained. Mottled rusty and gray colors are com- tion of a wide range of field and vegetable crops, mon at depths of 50 centimeters or less. Aquepts including sod. occur in many depressions in geologically recent landscapes where soils are water saturated at some time during the year, and are surrounded by better drained soils that are Alfisols or Mollisols. INCEPTISOLS Water saturation inhibits eluviation and illuvia- Inceptisols typically have an ochric or umbric epi- tion. With better drainage, Aquepts may develop pedon that overlies a cambic (Bw) horizon. They into soils that are older or more mature. Aquepts I NCEPTISOLS 301 can have any temperature regime and almost any Other large and important areas of Aquepts occur vegetation. in the world's major river valleys. Note the large The largest areas of Aquepts occur in the Tun- 12a area in Figure 18.1 that includes the lower dra regions of the Northern Hemisphere in Alaska, Mississippi River valley. The soils have formed in Canada, Europe, and Asia (areas 12a in Figure alluvial materials, but most of the materials are 18.2). Most of these cold Aquepts (Cryaquepts) not recent and the soils are not Entisols, because have permafrost, which inhibits the downward the river has wandered back and forth over a wide movement of water and contributes to soil wet- floodplain over a long period of time. These ness when frozen soil melts during the summer. Aquepts tend to be fine textured and very fertile. Trees grow poorly on soils with permafrost, so Good drainage and management have made the forestry is not important. Locally, above timber- Aquepts of the Mississippi River valley among the line in many mountains, there are Cryaquepts most productive soils in the United States. with permafrost (see Figure 18.12). Sugarcane is an important crop in southern Lou- Few people inhabit tundra regions; some of isiana where temperatures are warm. A landscape those who do work in the mining and oil indus- in which most of the soils are Aquepts in the lower tries, and a few people in northern Europe and Mississippi River valley is shown in Figure 18.13. Asia raise reindeer. The reindeer browse on the The Ganges-Brahmaputra River valley in north- tundra vegetation and may be readily contami- ern India and Bangladesh is the largest area in the nated with radioactive elements that fall out of the world in which Aquepts are the dominant soils atmosphere. Reindeer farmers in Finland found (see Area 12c of Figure 18.2). The presence of that their animals were unfit for human consump- Aquepts in the river valleys of Asia, and the large tion because of the radioactive contamination fol- rice consumption of the inhabitants makes lowing the Chernobyl (USSR) reactor explosion. Aquepts the world's most important soil for rice production. Aquepts are also the dominant soils in the Amazon River valley, (see Figure 18.2).

FIGURE 18.12 Dominant soils on the tundra above timberline in the mountains are Inceptisols, Aquepts. O Soils have permafrost, vegetation is sparse, and the Ochrepts are the more or less light-colored and environment is fragile. freely drained Inceptisols of the mid- to high latitudes. They tend to occur on geologically young land surfaces and have ochric and cambic horizons. Most of them had, or now have, a forest vegetation. Ochrepts are the dominant soils of the Appalachian Mountains (area 13s of Figure 18.1). Erosion on steep slopes has contributed to the formation and maintenance of these Inceptisols.

U Umbrepts are Inceptisols with umbric epipedons. Most of them occur in hilly or mountainous regions of high precipitation and under coniferous forest. Umbrepts in the United States are the most extensive near the Pacific Ocean in the states of 302 SOIL GEOGRAPHY AND LAND USE

FIGURE 18.13 Intensive agriculture on Aquepts in the lower Mississippi River valley.

Oregon and Washington (see areas 14S and l4a of Mollisols are considered to be some of the most Figure 18.1). These Umbrepts have a mesic tem- naturally fertile soils for agriculture. Where temperature regime and udic or xeric moisture re- perature and water are favorable, high grain yields gimes. Many of the soils are on steep sloopes that are easily obtained. The most abundant soils in are densely covered with coniferous forest and the United States are Mollisols, being 25 percent lumbering is the principal industry. of the total (see Table 18.1). Few Mollisols occur in the tropics because of the generally long period of weathering and leaching. The world's major grasslands lacked trees for MOLLISOLS lumber, readily available water supplies, and nat- Mollisols are the dominant soils of the temperate ural sites for protection against invaders. As a grasslands or steppes. These regions are fre- result, these areas dominated with Mollisols were quently bordered on the drier side by deserts with inhabited mainly by nomadic people until about Aridisols and on the wetter side by forests in 150 years ago. The farmers who settled in the which the dominant soils are Alfisols. There is central part of United States tended to avoid the sufficient precipitation to support perennial Mollisols because they were used to forest soils, grasses that contribute a large amount of organic and they found the sod of the Udolls difficult to matter to the soil annually by deeply pentrating plow and convert into cropland. The Virgin Lands roots. Precipitation, however, is somewhat lim- of the Soviet Union were opened to settled agricul- ited so the soils are only minimally or moderately ture in 1954. Today, these areas are characterized weathered and leached. Mollisols typically have a by their excellent agricultural soil and a low popu- mollic epipedon. Subsurface horizons are typi- lation density. Land use is related mainly to water cally cambic or argillic, and a few have natric supply and temperature. horizons. Mollisols are characterized by a moderate or high organic matter content and a high content of weatherable minerals. Many of the Aquolls soils are calcareous at the surface and the acid Aquolls are the wet Mollisols that have an aquic Mollisols tend to be only mildly acid and without moisture regime or are artificially drained. They significant exchangeable aluminum. Two Molli- occur as inclusions in many grassland land- sols, both with k (calcic) horizons, are shown in scapes owing to water saturation of the soil in Color Plate 6. local depressions. Drummer silty clay loam is an MOLLISOLS 303

Aquoll and is the most abundant soil in Illinois. Figure 18.1). Winter wheat, produced in a wheat- Much of the Corn Belt's reputation for corn pro- fallow system, is the major crop grown on Ustolls. duction is due to large acreages of Aquolls that Kansas ranks first in wheat production in the developed on nearly level and depressional areas United States. The winter wheat is planted in the under poor drainage. fall. The milder winters on Ustolls (as compared Aquolls are the dominant soils of the Red River with those on Borolls) allow the wheat to become Valley bordering Minnesota and North Dakota established in the fall and to begin growing in (area M1a in Figure 18.1). These Aquolls have early spring. Consequently, winter wheat yields formed from lacustrine sediments deposited in are greater than those for spring wheat. Spring glacial Lake Agassiz about 9,000 to 12,000 years wheat is grown primarily on Borolls of the north- ago. The soils tend to be fine-textured, to have a ern Great Plains in the United States and Canada, relatively high organic matter content, and to be and in the Soviet Union. Cattle grazing is also a well suited for grain crops. The Red River Valley is major enterprise on Ustolls, especially on land an area of large-scale cash crop farming. Spring not suitable for wheat (see Figure 18.14). Irri- wheat, barley, sugar beets, corn, and sunflowers gation water pumped from the Ogallala formation are the major crops. is used for intensive cropping on the High Plains of Texas. Common crops include sorghum, winter wheat, and peanuts. B Corn and soybeans are the major crops grown Borolls are Mollisols that do not have an aquic on Udolls, as shown in Figure 18.15. The two most moisture regime but are northerly or cool with famous areas of Udolls are the Corn Belt in the cryic temperature regime (have mean annual soil United States and the humid part of the Pampa in temperature less than 8° C or 47° F). Borolls are Argentina, which are areas of intensive corn pro- dominant on the northern Great Plains in the duction. There is limited production of corn in the United States and Canada and in the Soviet Union Soviet Union, which is a reflection of the cooler and northern China (areas M2a, b, and c in Figure and drier climate and the dominance of Ustolls 18.2). Spring wheat is the major crop and much of and Borolls rather than Udolls on the grasslands. the land is used for cattle grazing. Low tempera- Note in Figure 18.2 that there is a west to east ture is frequently coupled with low precipitation distribution of Aridisols, Ustolls, and Udolls in so that much of the wheat is produced in a wheat- fallow system.

FIGURE 18.14 Beef cattle grazing on Ustolls where the slopes are unfavorable for wheat production. Ustolls and Udolls (Courtesy USDA.) Ustolls and Udolls have warmer temperatures than do Borolls, and have ustic and udic moisture regimes, respectively. Ustolls typically have k horizons and a neutral or alkaline reaction. Udolls are without k layers. Udolls are typically acid, and lime is frequently applied. Cambic and argillic horizons are common in both. In both the Great Plains and the north-central states, Ustolls are dominant west of the approximate boundary between Iowa and Nebraska and Udolls are dominant east of the boundary. (Con- trast areas M4a, b, and c with the M3a areas in 304 SOIL GEOGRAPHY AND LAND USE

FIGURE 18.15 Typical Aquoll-Udoll landscape in central Illinois with corn and soybeans as the major crops.

Argentina, similar to that which occurs in the that overlies basalt in a rolling landscape (see United States. Land uses in the two areas are also Figure 18.16). The soils are very similar to Molli- similar, with wheat and cattle grazing on the sols developed from loess in the central United Ustolls and corn production concentrated on the States. The xeric moisture regime precludes corn Udolls. Most of the Mollisols of Argentina formed production because the summers are very dry. in loess that was contaminated with volcanic ash, Much of the land with Xeroll soils is used for cattle and therefore the soils have a greater inherent grazing. fertility than do comparable Mollisols in the United States and the Soviet Union. OXISOLS Xe Oxisols are the most intensively weathered soils, Xerolls are Mollisols with warmer temperatures consisting largely of mixtures of quartz, kaolinite, than are associated with Borolls and a xeric mois- oxides of iron and aluminum, and some organic ture regime. Most of the Xerolls in the United matter. Many Oxisols contain little silt because States occur in Washington, Oregon, and Idaho, the silt sized mineral particles have essentially where summers are dry and winters are cool and weathered leaving a clay fraction composed rainy. Water availability is suited for winter wheat mainly of oxidic clays plus kaolinte and varying production, and Whitford County in the state of amounts of sand because of the great weathering Washington produces more wheat than any other resistance of large quartz grains. Most Oxisols county in the United States. Whitford County is have oxic horizons. Some have plinthite, which located in the Palouse of eastern Washington and forms a continuous layer within 30 centimeters of western Idaho. The Xerolls developed in loess the soil surface and is water saturated at this OXISOLS 305

FIGURE 18.16 Landscape of Xerolls in the Palouse in Washington composed mainly of loessial dunes. Winter wheat is the major crop. (Photograph courtesy Dr. H. W. Smith.)

depth sometime during the year. Plinthite is an sive management, as in the case of pineapple iron-rich material that forms some distance below production in Hawaii, shown in Figure 18.17. the soil surface. When plinthite is exposed to There are no Oxisols within the continental drying, it dries irreversibly, forming a rocklike ma- United States; however, they are found in Hawaii terial. and Puerto Rico. As a result, Oxisols are the least Differences in properties with depth in Oxisols are so gradual that horizon boundaries are difficult to determine. A photograph of an Oxisol is FIGURE 18.17 Pineapple production on Oxisols in shown in Color Plate 6. Hawaii. Oxisols are characterized by a very low amount of exchangeable nutrient elements, very low content of weatherable minerals, high aluminum saturation of the cation exchange capacity, and moderate acidity. Although aluminum saturation may be high, the crops do not generally develop aluminum toxicity because of the low CEC. The mineralogy contributes to very stable peds or structure that results in qualities associated with sandy, quartzitic soils in the temperate regions. Gener- ally, Oxisols represent the most naturally infertile soils for agriculture. Some are, however, the world's most productive soils because of inten- 306 SOIL GEOGRAPHY AND LAND USE abundant soils in United States. Worldwide, Oxi- sols are the sixth most abundant soils; however, Oxisols are the most abundant soils of the tropics, where they occur on about 22 percent of the land.

O S Oxisols tend to occur along or parallel to the equator. Near the equator, where there is significant rainfall each month, many Udox and Perox soils have developed. Udox have udic moisture FIGURE 18.18 Preparation of land in the foreground by slash and burn. Note stumps left in field and the regime and Perox have perudic moisture regime. considerable amount of organic matter on the soil North and south of the equator are regions in surface that will mineralize and provide plant which there is abundant rain in summer but none nutrients. in winter. In these areas, the Oxisols that develop have an ustic moisture regime and are Ustox. surface and the dense rain forest growth in the About two thirds of the Oxisols are Udox (and background. Perox) and about one third are Ustox. Oxisols are Frequently, a village or tribal chief allocates dominant soils in two large areas. One is in the land to the families. Vegetables grown in small Amazon basin in South America and the other is gardens near houses, and fish and game animals centered on the Congo River or Zaire basin in supplement the diet. There are no farm animals to central Africa. Note the distribution of Udox and provide manure for the maintenance of soil fertil- the Ustox soils by comparing the location of the ity, and only a sparse population can be sup- Udox areas, O1b , with the Ustox areas, 02b, in ported. Where population growth occurs, shorter Figure 18.2. Oxisols of the Aquox and Torrox fallow periods (periods when the forest is regrow- suborders are of minor extent. ing and accumulating nutrients within plant and soil organic matter) are used and it may become impossible to regenerate soil fertility. Then, the L U U S system becomes ineffective. The accumulation of The Oxisols in the year-round rainy climates, nutrients in the vegetation during forest fallow in Udox, are mainly in the tropical rain forests where Zaire (Congo) is shown in Table 18.3. The 18- to a small human population is supported by shifting cultivation or slash-and-burn agriculture. Trees are killed by girdling and/or burning, and TABLE 18.3 Nutrient Accumulation in Forest Fallow the ashes and decomposing plant remains provide nutrients for crops for a year or two. Then, soil fertility exhaustion and weed invasion result in abandonment of the land, which is gradually reforested by invasion of trees and shrubs. After a period of about 20 years, sufficient nutrients will have accumulated within living and dead plant materials and in soil organic matter to allow for another short period of cropping. Land prepared Adapted from C.E., Kellogg, "Shifting Cultivation," Soil Sci., for growing crops is shown in Figure 18.18. Note 95:221-230, 1963. Used by permission of Williams and Wat- the large amount of organic residues on the soil kins Co. OXISOLS 307

19-year-old forest vegetation contained five times more nutrients than were contained in the 2-year- old forest fallow. Forests grow vigorously on Oxi- sols in the humid tropics because water is generally available and (1) plant nutrients are efficiently cycled and (2) deeply penetrating roots absorb nutrients from soil layers that are much less weathered and more fertile than the A and B horizons.

L U U S The native vegetation of the savanna areas, where Ustox occur, is not suited for the type of slash- and-burn agriculture that occurs in the rain forests FIGURE 18.20 Land preparation of Ustox soils on on Udox soils (see Figure 18.19). There are vari- the African savanna. Organic refuse, including any ous kinds of fallow systems used. However, the animal manure, is placed in the furrows between the opportunity to accumulate nutrients in plant ma- ridges. Then, each ridge is split and the soil is pulled terial is much less in the savanna than in the rain into the furrow to form a new ridge where the furrow previously existed. Ridged land increases water forest. Farmers in these areas tend to be small infiltration and reduces erosion from torrential subsistence growers who cultivate a small summer storms. acreage permanently and give greater attention to the use of manures, plant residues, and fertilizers to maintain soil fertility. Periods of 6 months with throughout the savanna and provide some food little if any rainfall are common, and each family with minimal care. Corn, beans, cowpeas, and tends to farm a small acreage of wetland, (if it is cassava are important crops. Some farmers have available), where vegetables are harvested during large herds of cattle. A farmer is shown preparing the dry season. Many fruit trees are scattered land for cropping on Ustox in Africa in Figure 18.20.

FIGURE 18.19 Natural vegetation on savannas in Brazil, consisting of small trees, shrubs, and grass in E W O an area dominated by Ustox soils. The maximum effective cation exchange capacity (ECEC) of the clay fraction of oxic horizons is 12 or less cmol/kg. By contrast, the ECEC of the most weathered horizons in Oxisols is 1.5 or less cmol/ kg. On this basis, soils with a horizon having an ECEC of 12.5 to 1.5 cmol/kg are considered to be intensively weathered and soils with a horizon having an ECEC of 1.5 or less cmol/kg are considered to be extremely weathered. The prefix acr is used to indicate this low ECEC as in the case of Acrudox, extremely weathered Udox. The horizon with acricity must be 18 or more centimeters thick. 308 SOIL GEOGRAPHY AND LAND USE

The pH of soils is closely related to the kinds of also form at the base of slopes where water seeps exchangeable cations. Extremely weathered soils out at the soil surface. Plinthite is soft to the extent with very low ECEC have a small capacity to de- that it can be cut with a spade and readily mined. velop low pH values even when nearly 100 per- On exposure to the sun, the material hardens to a cent aluminum saturated and tend to have a pH bricklike substance, which is used as a building that is greater than that for intensively weathered material (see Figure 18.21). soils. Even though extremely weathered soils may There is great diversity in plinthite, ranging be essentially 100 percent aluminum saturated, from continuous thick layers to thin, discontinu- there may be insufficient A1 3+ in solution to pro- ous nodular forms. Small amounts of nodular duce aluminum toxicity. Such soils have very plinthite or deeply buried, continuous layers be- small amounts of exchangeable calcium and low the root zone have little influence on plant magnesium. Lime is used in very small quantities growth. Where continuous plinthite is exposed by to supply calcium and magnesium with minimal erosion, drying and hardening can seriously limit effect on soil pH. plant growth. As long as the layer remains moist Horizons in extremely weathered soils that con- and soft, and is below the root zone, soils can be tain very little organic matter sometimes have a effectively used for a wide variety of crops. In net charge of zero or a net positive charge. Soils some places holes are dug in hardened plinthite with a horizon 18 or more centimeters thick that and then filled with soil to grow high-value crops. has a net charge of zero or a net positive charge It is estimated that hardened plinthite, ironstone are considered anionic. Thus, an Acrudox with an or laterite, occurs on 2 percent of the land in anionic horizon is an Anionic Acrudox. tropical America, 5 percent in Brazil, 7 percent in the tropical part of the Indian subcontinent, and 15 percent for Sub-Saharan West Africa. P L A soil horizon associated with the tropics and many Oxisols, and also with Ultisols and Alfisols, SPODOSOLS is plinthite. It is an iron-rich mixture containing quartz and other dilutents that remains soft as Spodosols contain spodic horizons in which long as it is not dried. On exposure at the soil amorphous illuvial materials, including organic surface, due to erosion, the material dries and matter and oxides of aluminum and iron, have hardens irreversibly with repeated wetting and accumulated. Quartzitic sand-textured parent ma- drying. The hard, rocklike material is hardened terial and a humid climate with intense leaching plinthite and is commonly called ironstone or promote development of spodic horizons. As a laterite. result, Spodosols are widely distributed from the Plinthite can, perhaps, form from the concen- tropics to the tundra. Trees are the common vege- tration of iron in situ, but it more likely forms via tation. the influx of iron in moving groundwater. Plinthite formation is characteristically associated with a fluctuating water table in areas having a short dry S S season. Reduced iron (ferrous), produced in In the United States, about 86 percent of the Spo- water-saturated soil, is much more mobile than dosols are Orthods and the remainder are oxidized iron (ferric) and moves in water and Aquods. Aquods have aquic moisture regime and precipitates on contact with a good supply of oxy- are the dominant soils in the state of Florida, gen. Plinthite normally forms as a subsurface which has abundant sand-textured parent mate- layer that is saturated at some season, but it can rial, abundant rainfall, and shallow water tables

SPODOSOLS 309

FIGURE 18.21 Mining plinthite in Orissa state, India. On drying, the bricks harden and are used for construction.

that are close to the soil surface at least part of the Suborders of minor extent include Humods and year (see the S1a areas for Florida in Figure 18.1). Ferrods with a relatively high organic matter and The two major kinds of soils in Florida are both iron content in the spodic horizons, respectively. very sandy and naturally infertile-Aquods and Psamments. The warm climate, together with intensive soil management, makes these soils pro- S P L U ductive for winter vegetables, citrus crops, and Spodosols typically have a very low clay content year-round pastures. in all horizons; less than 5 percent for the soil Orthods are the most common Spodosols; an horizons cited in Table 18.4. Sola are quite acid, Orthod is shown in Color Plate 5. They are abun- with a pH less than 5 being very common. There is dant in the northeastern United States where they a small cation exchange capacity, ranging from have developed in late Pleistocene sandy parent 1.3 cmol/kg (meq/100 g) for the C horizon to 5.2 materials (see areas S2a in Figure 18.1). They cmol/kg (meq/100 g) for the Bhs horizon. The occur in association with Alfisols in the Great cation exchange capacity of each horizon is Lake states where the Spodosols have developed closely related to the amount of amorphous ma- on sandy parent materials and the Alfisols have terial-greater in the spodic horizon (Bhs) than in developed on the loamy parent materials. The soil the E or lower B and C horizons. There is a modest boundary line across lower Michigan and north- aluminum saturation of the cation exchange ca- ern Wisconsin is, in essence, a parent material pacity which, coupled with a low cation exchange boundary. The boundary separates an area with capacity, results in a very low quantity of ex- loamy Alfisols to the south from an area to the changeable calcium, magnesium, and potassium north dominated by sandy Spodosols. This pat- (see Table 18.4). These properties contribute to tern is repeated across northern Europe and Asia, soils that are too infertile for general agriculture as shown in Figure 18.2. and have a low water-holding capacity. 31 0 SOIL GEOGRAPHY AND LAND USE

TABLE 18.4 Selected Properties of a Spodosol (Orthod)

Trace amount. Selected data for pedon 25 in Si Ta, 1975.

The natural infertility and droughtiness of Spo- port extractive agriculture which takes nutrients dosols has been cited as an example of a case out of the soil and does not replace them. Many where virgin soils are not necessarily fertile and New England lands that were treated in this way suitable for agriculture. The following quote is soon went back to forest. from an article that appeared in Time magazine in 1954: In 1870, the widespread conversion of forest land in New Hampshire peaked to about 75 per- The virgin soil under a long-established forest is cent improved farmland and then rapidly de- not always good. When the settlers cleared New creased to only about 10 percent improved farm- England forests 300 years ago, the topsoil they land by 1930, as shown in Figure 18.22. As found was only 2 to 3 inches thick. Below this was settlement moved westward, the plow followed sterile subsoil and when the plow mixed the two the cutting of the forest and a similar land-use together, the blend was low in nearly everything a pattern also occurred on the Spodosols in the good soil should have. It was not the lavish virgin Great Lake states. The first bulletin published by soil of popular fancy. Such a soil could not sup- the Michigan Agricultural Experiment Station in 1885 was devoted to solving problems on the sand plains of north-central Michigan. The abandonment of land for agricultural use where soils FIGURE 18.22 Percent of improved farmland in are Spodosols is shown in Figure 18.23. New Hampshire from 1790 to 1970. (Courtesy Steven The cool summers of the northern Spodosol Hamburg, Yale University.) region attract many tourists. Many cities in this area owe their existence to mining and lumbering. Although agriculture is of minor importance, there are localized areas of intense fruit, vegetable, and dairy farming. This farming occurs on two kinds of soils. In one kind the agriculture occurs on inclusions of Alfisols within the Spo- dosol region. These are situations where the parent materials are loam-textured and the Alfisols are Boralfs. In the other kind, the soils have a sandy upper Spodosol solum that overlies a horizon of increased clay content-a Bt (argillic) horizon. These are bisequum soils that contain the E ULTISOLS 31 1

FIGURE 18.23 These disintegrating buildings tell the story of forest removal, farming, and then abandonment of land dominated by Spodosols in the Great Lake States. and Bhs horizons of a Spodosol that typically southeastern United States. The water tables are overlie some E and the argillic horizon of an Al- naturally close to the soil surface at some time fisol. These are the dominant soils in the well- during the year. Further inland, at higher eleva- known potato-growing area in Aroostook County, tions on the middle and upper coastal plains, the Maine and in adjoining New Brunswick, Canada. Ultisols are mainly Udults. Contrast areas U1a (Aquults) with U3a (Udults) in Figure 18.1. Hum- ults are found in the coastal ranges of northern California, Oregon, and Washington (area U2s in ULTISOLS Figure 18.1). Humults have more than the usual The central concept of Ultisols is ultimately amount of organic matter and develop under con- leached soils of the mid- to low latitudes that have ditions of high precipitation and cool tempera- a kandic or an argillic horizon but few weathera- tures, associated with mountain slopes. ble minerals. In parent materials with almost no A large region of mostly Udult soils is found in weatherable minerals, a Ultisol may develop in a southeastern China at a latitude and continental few thousand years. Perhaps, 100,000 years or position similar to that of the southeastern United more are required if the Ultisol develops from the States (see U3d in Figure 18.2). Many smaller weathering and leaching of a former Alfisol. Ulti- areas of Ultisols occur in Africa and South sols are the dominant soils of the Piedmont and America, usually in association with Oxisols. coastal plain of the southeastern United States beyond the area of glaciation and young parent P U materials. The large Ultisol region extends from A common horizon sequence for many Ultisols is Washington, D.C., south and westward into east- A, E, Bt and C. Ultisols that develop in iron-rich ern Texas, as shown by the U3a areas in Figure parent materials may have intense red colors and 18.1. Ultisols are the fourth most abundant soils in lack an obvious E horizon. A photograph of a the United States (see Table 18.1) and the fourth Udult is shown in Color Plate 5. The symbol, v, as most abundant soils in the tropics, following Oxi- in Btv, indicates plinthite, which in this instance is sols, Aridisols, and Alfisols. not continuous and has mixed red and yellow color (see the Ultisol in Color Plate 5). U S In the United States, 76 percent of the Ultisols Aquults have an aquic moisture regime and occur are Udults. The horizons of Udults are quite acid extensively along the lower coastal plain of the to a great depth and they contain a very small 31 2 SOIL GEOGRAPHY AND LAND USE

amount of exchangeable calcium, magnesium, those of his neighbors that they averaged only 10 potassium, and sodium. The very low amounts of bushels of corn per acre and even the better lands these exchangeable cations reflect the low con- a mere 6 bushels of wheat. Ruffin happened to tent of weatherable minerals-less than 10 per- read Elements of Agricultural Chemistry by Sir cent in the coarse silt and fine sand fractions. Humphrey Davy, from which he learned that Leaching removes cations from the soil more rap- acidity was making the soils sterile, and liming idly than they are released by weathering. Ultisols was needed to neutralize soil acidity. The applica- are required to have less than 35 percent of the tion of marl to the land was very successful. Now, cation exchange capacity, which is determined at it is known that the marl (or liming) added cal- pH 8.2, saturated with calcium, magnesium, po- cium and magnesium and reduced aluminum tassium, and sodium in the lower part of the root toxicity. Liming and the increase in soil pH made zone (at a depth of about 125 centimeters below the application of manure and use of legumes in the upper boundary of the argillic horizon or 180 rotations more successful. For this discovery, Ruf- centimeters below the soil surface). By contrast, fin was credited with bringing about an agricul- the exchangeable aluminum is high and roots tural revival in the South. With the long, year- may grow poorly in subsoils because of an alumi- round growing season and abundant rainfall, this num toxicity. Many subsoils contain too little cal- region that once proved so troublesome is now cium for good root growth. Most of the plant nutri- one of the most productive agricultural and for- ent cations are bound within the plants and/or estry regions in the United States (see Figure soil organic matter. Ultisols are naturally too infer- 18.24). tile for the production of many agricultural crops Land use on Ultisols in the tropics is quite simi- because of their acidity, low content of exchange- lar to land use on nearby Oxisols. S1ash-and-burn able basic cations, and the potential for alumi- and subsistence agriculture are common. Many num toxicity. plantations or estates produce tea, rubber, bana- nas, tobacco, and other valuable crops.

Land Use on Ultisols During the early settlement of United States, agriculture on Ultisols in Virginia and the surrounding VERTISOLS states consisted mainly of tobacco and cotton Vertisols are mineral soils that are more than 50 production for the European market. After centimeters thick, contain 30 percent or more clay clearing the forest and cropping for only a few in all horizons, and have cracks at least 1 cen- years, the naturally infertile soils became ex- timeter wide to a depth of 50 centimeters, unless hausted. Abandonment of land and a shifting type irrigated at some time, in most years. The typi- of cultivation became widespread as the frontier cal vegetation in natural areas is grass or herba- moved westward. This was the plight of early Vir- ceous annuals, although some Vertisols sup- ginians. One farmer in particular, Edmund Ruffin, port drought-tolerant woody plants. described the situation as follows: Worldwide, and in United States, Vertisols are the ninth most abundant soils. The three largest Virginians by the thousands, seeing only a dismal areas are in Australia (70 million acres), India (60 future at home, emigrated to the newer states of million acres), and the Sudan (40 million acres). the South and Middle West. The population These soils are mainly Usterts, and are shown as growth dropped from 38 percent in 1820 to less areas of V2a and V2c in Figure 18.2. than 14 percent in 1830, and then to a mere two Vertisols in the United States have formed percent in 1840. All wished to sell, none to buy. mainly in marine clay sediments on the coastal So poor and exhausted were Ruffin's lands and plains of Texas, Alabama, and Mississippi. Very VERTISOLS 31 3

FIGURE 18.24 Udults on the coastal plain in the southeastern United States have nearly level slopes and sandy surface soils that are easily tilled.

sharp boundaries between soils of different or- weather to form, a large amount of expanding ders may occur in the area because Vertisols de- clay, usually smectites; and (2) a climate with veloped on clay sediments, whereas Ultisols de- alternating wet and dry seasons. After the cracks veloped on adjacent sandy sediments. develop in the dry season, surface soil material and material along the crack faces, fall to the bottom of the cracks. The soil rewets at the begin- V S ning of the rainy season from water that quickly Approximately 40 percent of the Vertisols in runs into and to the bottom of the cracks, which United States are Uderts and 60 percent are causes the soil to wet from the bottom up during Usterts. Usterts are sufficiently dry so that cracks the early part of the wet season. After the cracks at remain open more than 90 cumulative days in a the surface close, the soil wets normally from the year. There are two areas in Texas where Usterts surface downward. This results in a dry soil layer are the dominant soils (areas V2a in Figure 18.1). sandwiched between two layers that are being Uderts have a wetter climate than do Usterts and wetted and undergoing expansion. Thus, there is have cracks that are open 90 or fewer cumulative considerable moving and sliding of adjacent soil days in a year. A large area of Uderts occurs along masses and the formation of shiny ped surfaces- the Gulf Coast of Texas (V1a in Figure 18.1). A slickensides. A large slickenside is shown near smaller area occurs along the border of Alabama the AC label of the Vertisol in Color Plate 5. and Mississippi. Torrerts and Xererts occur to a Expansion or swelling of soil in the vicinity of lesser extent. cracks results in great lateral and upward pressures, which cause a slow gradual movement of soil upward between areas that crack. This soil V G movement produces a microrelief called gilgai Conditions that give rise to the formation of Verti- (see Figure 18.25). The major features of Vertisols sols are: (1) parent materials high in, or that are illustrated in Figure 18.26. 31 4 SOIL GEOGRAPHY AND LAND USE

FIGURE 18.25 Gilgai microrelief on Vertisols in Texas. (Photograph Soil Conservation Service, USDA.)

V P tent, but the organic matter decreases gradually The high content of swelling clay and movement with increasing soil depth. This pattern of organic of soil by expansion and contraction retard the matter content is similar to that which occurs in development of B horizons. Typically, the soil many soils; in Vertisols, the change in organic remains quite undeveloped and often has an A-C matter content may be associated with no change profile. Some properties of the Houston Black in color (see Vertisol in Color Plate 5). This Verti- clay from the Blackland Prairie region of Texas are sol contains some CaCO 3 and has a pH of about presented in Table 18.5. There is a high and con- 8.3 in all horizons. Some Vertisols, Uderts, are sistent clay content in each horizon. Organic mat- sufficiently leached to be acid in some horizons. ter content is typical for soils with high clay con- The soil has high fertility for many agricultural

! FIGURE 18.26 Schematic drawing showing a soil profile of Vertisols. Major features include deep cracks, parallelepiped structure (rhombohedral aggregates), slickensides, and gilgai microrelief basin. (Adapted from Dudal and Eswaran, 1988.)

Slickensides VERTISOLS 315

Table is based on data of Kunze and Templin, 1956. Average of 5 profiles. crops and the soils can retain a large amount of the crops depended primarily on stored soil wa- water. Conversely, when the soils are wet the inter. Some creative research done at the Interna- filtration of water is essentially zero, which results tional Crops Research Institute for the Semiarid in excessive water runoff and soil erosion on slop- Tropics (ICRISAT) in Hyderabad, India, resulted ing cultivated land. in recommendations about some different tillage methods. Tillage was to be done immediately after harvest of the winter crops before the soils L U V became too dry to till. Some occasional tillage It is difficult for rainwater to move through Verti- during the dry season was also recommended. sols and contribute to a groundwater aquifer. Then, farmers could establish the crop in loose, Those who settled on Vertisols had difficulty dry soil before the monsoon rains began. The drilling wells and obtaining water. For this reason, and because many Vertisols occur in regions of limited rainfall where moisture regimes are ustic, FIGURE 18.27 Post-monsoon crop of cotton on many Vertisols have been minimally or only mod- Usterts in India. Cotton is a major crop, which is why erately leached. Thus, Vertisols are considered the Usterts are called Black-Cotton soils. Note the fertile for many agricultural crops (see Figure many slickensides surfaces and black color to a 18.27). depth of more than a meter. Most soil management problems are the results of the high content of expanding clay and accompanying physical properties. In Texas, the Verti- sols were referred to as "dinner-pail-land," being too wet and sticky to till before the noontime meal (dinner) and too dry and hard to till after the noontime meal. Farmers in India found the Usterts too hard to till with their small plows and oxen at the end of a long dry season. They were unable to plant crops before the monsoon rains started, which meant that crops were not produced during the season when water is most available. Then, crops were planted after the moonsoon rains stopped, and 31 6 SOIL GEOGRAPHY AND LAND USE

FIGURE 18.28 Structural failure caused by landslide on Vertisols produced by wet soil sliding downhill. Note that pipes are on top of the ground to prevent breaking caused by expansion and contraction of soil.

result was high-yielding crops during the mon- ejecta. Spodosols develop from sand parent mate- soon season (in addition to the post-monsoon rial in humid regions and are droughty, acid, and crops), and more than 100 percent more total infertile. Vertisols develop in clay materials that production per year as compared with the tradi- expand greatly with wetting and shrink with dry- tional system. ing. They are frequently minimally weathered and Open cracks are a hazard for grazing animals leached, and are fertile soils for cropping. Verti- during dry seasons. The expansion and contrac- sols have unique use problems associated with tion of soil cause a misalignment of fence and their clayey nature. telephone posts, breaks pipelines, and destroys Aridisols, Entisols, and Inceptisols typically are road and building foundations as shown in Figure minimally weathered soils that tend to be rich in 18.28. primary minerals and to be fertile for cropping. Some Inceptisols in tropical regions, however, have developed in intensively weathered parent SUMMARY material and are acid and infertile. Moderately weathered and generally fertile Histosols are soils formed from organic soil mate- soils include Alfisols and Mollisols. These soils rials. Typically, they are water saturated for part of are common in the temperate regions and are the year. Drainage is required for cropping, but generally good for agricultural use. subsequently, decomposition of the organic mat- Ultisols and Oxisols are intensively weathered ter results in a gradual disappearence of the soil soils. They are generally acid and infertile, and and subsidence of the soil surface. aluminum is toxic for many plants. Many are used Andisols, Spodosols and Vertisols owe their for slash-and-burn agriculture and subsistence existence, largely to the parent material factor of farming. Intensive management can result in pro- soil formation. Andisols develop in volcanic ductive soils with high crop yields. REFERENCES 31 7

REFERENCES Kellogg. C. E. 1963. "Shifting Cultivation." Soil Sci. 95:221-230. Kunze, G. W. and E. H. Templin. 1956. "Houston Black Buol, S. W. 1966. "Soils of Arizona."Ariz. Agr. Exp. Sta. Clay, the Type Grumusol: II Mineralogical and Chem- Tech. Bull. 117, Tuscon. ical Characterization." Soil Sci. Soc. Am. Proc. 20: Dudal, R. and H. Eswaran. 1988. "Distribution, Proper- 91-96. ties, and Classification of Vertisols," in Vertisols: Ruffin, Edmund. 1961. An Essay on Calcareous Ma- Their Distribution, Properties, Classification, and nures. Cambridge, Mass. (Reprint of original book Management. L. P. Wilding and R. Puentes (eds.) that was published in 1832). Texas A & M University Printing Center, College Sanchez, P. A. 1976. Properties and Management of Station. Soils in the Tropics. Wiley, New York. Foth, H. D. and J. W. Schafer. 1980. Soil Geography and Soil Survey Staff. 1975. "Soil Taxonomy." USDA Agr. Land Use. Wiley, New York. Handbook 436. Washington, D.C. Kanwar, J. S., J. Kampen, and S. M. Virmani. 1982. Soil Survey Staff. 1987. "Keys to Soil Taxonomy." Soil "Management of Vertisols for Maximizing Crop Pro- Management Support Services Tech. Monograph 6. duction-ICRISAT Experience." Vertisols and Rice Cornell University, Ithaca, New York. Soils of The Tropics, Symposia Papers f/. pp. 94-118. Swanson, C. L. W. 1954. "The Road to Fertility." Time, 12th Int. Congress Soil Sci., New Delhi. January 18. Kedzie, R. C. 1885. "Early Amber Cane as a Forage Tuan, Yi-Fu. 1969. China. Aldine, Chicago. Crop." Agr. College of Michigan Bull. 1. East Lan- Young, A. 1976. Tropical Soils and Soil Survey. Cam- sing. bridge, London. CHAPTER 19

SOIL SURVEYS AND LAND USE INTERPRETATIONS

A soil survey is the systematic examination, de- soils. An aerial photograph is used as the map scription, classification, and mapping of soils in base. The soil surveyor walks over the land and an area. The kinds of soil in the survey area are simultaneously studies the aerial photograph and identified and their extent is shown on a map. The the landscape. This includes observations of soil map, together with an accompanying report ground cover or vegetation, slope, and landforms. that describes, defines, classifies, and makes in- In a glacial landscape, outwash plains will have a terpretations for various uses of the soils, is pub- nearly level surface and typically contain consid- lished as a Soil Survey Report. erable sand and gravel. Lake beds are nearly level with fine-textured soils. Ground moraines have an irregular surface with much variation in slope and texture. These three different landforms will likely MAKING A SOIL SURVEY have different plant species in the native vegeta- A soil survey requires making soil maps of the tion or, if in a cultivated field, have growth differ- area and writing an informational and interpreta- ences of crop plants. These three different situa- tive report. In many states, a soil survey is a coop- tions (outwash plain, lacustrine plain, ground erative activity between the U. S. Department of moraine)in a field will result in at least three dif- Agriculture and state Agricultural Experiment ferent kinds of soil. At selected locations, the soil Stations. The size of the area surveyed and the surveyor uses a soil auger to bore a hole and intensity of mapping depend on the amount of observe the sequential horizons of the soil profile detail required to make the necessary land-use as the bore hole samples of horizons are extracted interpretations. (see Figure 19.1). Observations of each horizon include: thickness, color, texture, structure, pH, presence or M S M absence of carbonates, and so on. In addition, the Soil maps are made by scientists who are familiar slope and degree of erosion are noted. This infor- with the local soil-forming factors, an understand- mation is then recorded directly on the aerial pho- ing of soil genesis, and the ability to identify the tograph, and lines are drawn to separate different

318 MAKING A SOIL SURVEY 31 9

FIGURE 19.2 Aerial photograph of a field in Ingham County, Michigan, where soils are Alfisols and Histosols. Standing at point X and looking north a person would observe a black area of Histosols sandwiched between areas of light-colored soils that are Alfisols. FIGURE 19.1 A soil surveyor indentifies the soil by inspecting the soil horizons. Lines are then drawn around areas of similar soil on an aerial photograph to produce a soil map. (Photograph courtesy USDA.) ganic soils (Histosols) and appear as a dark area on the aerial photograph. Beyond the depression, the soils are again well-drained Alfisols on a slope soil areas (mapping units) to create the soil map. similar to that at point X. From observations of the Other information is also recorded, such as loca- aerial photograph and landscape, the soil sur- tion of gravel pits, farmsteads, cemeteries, dams, veyor constructs a mental image of what kind of and lakes. soil is located in each part of the landscape. Then, Areas of different kinds of soil typically appear at selected locations, soil borings are made to on the aerial photograph as areas with different verify and identify the soil. The result is that the intensity of black and white, as shown in Figure soil surveyor does not randomly bore holes and 19.2. A large black area of Histosols is sand- make observations, but instead uses the auger to wiched between two light-colored areas of Al- confirm the hypotheses. This results in the boring fisols in a recently cultivated field. A soil surveyor of holes at strategic locations and the construc- standing at point X in Figure 19.2 would see the tion of the soil map with minimum physical input. landscape in late summer after grain harvest, as The soil map made for the recently cultivated shown in Figure 19.3. It is obvious in Figure 19.3 field (without a vegetative cover in Figure 19.2), is that at point X (Figures 19.2 and 19.3) the soil is shown in Figure 19.4. The symbols on the map on a slope. The soils near X are well-drained refer to the mapping units. Typically, the mapping mineral soils that have been subjected to erosion. unit contains three symbols such as 3B1-3 for These soils have a grayish-brown surface color soil, B for slope, and I for erosion. In this case it and appear as a light-colored area on the aerial means: 3 for Marlette sandy loam, B for a 2- to photograph. These soils are Alfisols. The slope 6-percent slope, and 1 for slightly eroded. Some ends in a depression in which the soils are very soils are always located on level or nearly level poorly drained. These black-colored soils are or- areas that are not eroded, and the slope and ero-

320 SOIL SURVEYS AND LAND-USE INTERPRETATIONS

FIGURE 19.3 On-the-ground view of the landscape from X of Figure 19.2 and looking north- northwest. The soil differences shown on the aerial photograph are also apparent after wheat harvest. The dark-colored area of Histosols in the depression area can be seen to be sandwiched between the Alfisol areas that are sloping.

sion notations are omitted. The mapping symbols for the soil map in Figure 19.4 are as follows: IA Capac loam, 0 to 3 percent slopes 2 Colwood-Brookston loam 3B1 Marlette sandy loam, 2- to 6-percent slope, slight erosion 4 Palms muck 5B Spinks loamy sand, 0- to 6-percent slope 6A Urban land, Capac-Colwood complex, 0- to 4-percent slope The mapping symbol at point X in Figures 19.2 and 19.3-3B1-is for the Marlette soil, which is a Udalf. In the center of the depressional area, the soil is Palms muck (4)-a Saprist. These two soils have very different properties, including slope, erosion, and drainage characteristics and would demand very different management regardless of whether they are used for agriculture, forestry, or for a home-building site.

1A Capac loam, 0-3 percent slopes 2 Colwood-Brookston loams 3B1 Marlette sandy loam, 2-6 percent slopes W S S R 4 Palms muck Typically, soil survey reports are for a county, 5B Spinks lomy sand, 0-6 percent slopes 6A Urban land, Capac-Colwood complex, 0-4 percent slopes parish, or another governmental unit. The soil MAKING A SOIL SURVEY 321

maps usually form the latter part of the report. of the soils for the mapping units in the front Anyone desiring information on a specific parcel portion of the report to learn about the general of land can refer to the appropriate soil map sheet features of these soils. This includes the kinds of and determine the kinds of soils. The first part of horizons and their properties and the conditions the report is written and consists of information under which the soils developed. For example, about the genesis and classification of the soils, the 4 mapping unit includes Palms muck, which descriptions and properties of each soil, and in- consists of very poorly drained organic soil about terpretative tables for various soil uses. 36 inches thick that overlies moderately coarse to fine-textured deposits. U S S R Factors that are important for building sites in- Imagine that you are driving down the road along clude natural drainage, depth to water table, the eastern side of the cultivated field shown in septic-field suitability (where municipal sewage Figure 14.2, because you are looking for a site on disposal is not available), supporting ability, which to build a house in the country. Would you shrink-swell potential, slope, and lawn and land- be able to find a suitable parcel of land in that scaping suitability. Some hazards and suitabilities field to use as a home site? How could the soil of the various mapping units, relative to construct- survey report help you in making your decision? ing a building with a basement, are given in Table To find the answers to these questions you 19.1. The Palms soils (4) have severe problems would need the Soil Survey Report for Ingham related to flooding and wetness. The water table is County, Michigan. First, locate in the soil survey at or within a foot of the soil surface from Novem- report the appropriate soil map in the back sec- ber to May. These conditions make the use of a tion and identify the soils within the parcel of land septic filter field for sewage effluent disposal im- that interests you. Then, refer to the descriptions possible because of the high water table. A dry

TABLE 19.1 L and-Use Intepretations 322 SOIL SURVEYS AND LAND-USE INTERPRETATIONS basement would be a virtual impossibility. Driveways and sidewalks, without footings or support on the mineral soil under the muck, would subside as the muck dries out and decomposes. Palms soil is obviously poorly suited for a building site. From Table 19.1, the soil with the best overall suitability is 5B (Spinks loamy sand). It is a well- drained and moderately to rapidly permeable soil. The water table is below 6 feet throughout the year, and the soil has sufficient water permeability for the septic-filter field and only moderate limita- FIGURE 19.5 Sites selected for solid waste disposal tions for lawn and landscaping. Spinks soils (landfills) need water-impermeable underlying strata would also have a low shrink-swell potential. to minimize groundwater pollution. A precaution is in order. Soils do not occur as uniform bodies. Mapping units typically contain 15 percent or more inclusions. That is, an area labeled 5B on a soil map could contain a signifi- Examples of Interpretative cant amount of similar but different soils. Thus, to Land-Use Maps establish whether or not a septic-sewage disposal All soils that are predicted to perform similarly for system will operate successfully in a particular a particular use can be grouped together, and an soil mapping area, a percolation test is needed for interpretative land-use map can be constructed. that small area within the mapping unit where the filter field will be located. FIGURE 19.6 Soil suitablility map for buildings with basements of the recently cultivated field shown in SOIL SURVEY INTERPRETATIONS AND Figure 19.2 (also in Figure 19.4). LAND USE PLANNING When a soil survey is made, there are specific needs to be fullfilled by the survey. The writers of the soil survey report include many tables of soil properties and characteristics, together with tables that contain predictions of soil behavior for many uses. We have used this type of soil survey information in regard to selection of a building site for a house in the country. Estimates of crop yields and rates of forest growth are useful for prospective land buyers and land tax assessors. Highway planners need information about flooding and wetness conditions and the ability of soils to support roads. Landfills for waste disposal require underlying layers that are impermeable to water (see Figure 19.5). Thus, land-use interpretation maps are needed to satisfy the requirements of various land users. SOIL SURVEY INTERPRETATIONS AND LAND-USE PLANNING 323

FIGURE 19.7 Soil survey map on the left was used to construct a wildlife suitability map on the right for a 40-acre tract of land. (Based on information from Allan, et al., 1963.)

The suitability map for construction of buildings in terms of erosion, wetness, rooting depth, and with basements in Figure 19.6 is based on the soil climate. Class VIII land, by contrast, precludes map of Figure 19.4 and the interpretations given in normal agricultural production and restricts use Table 19.1. to recreation, wildlife, water supply, or esthetic Soils directly affect the kind and amount of purposes. A land-use capability class map for a vegetation available to wildlife as food and cover, 200-acre dairy farm is Wisconsin in shown in Fig- and the soils affect the construction of water im- ure 19.8. poundments. The kind and abundance of wildlife that populate an area depend largely on the amount and distribution of food, cover, and wa- C S ter. Figure 19.7 shows a soil map and an accom- S I panying suitability map for the production of grain One of the major problems with the use of Soil and seed crops for wildlife use. The requirements Survey Reports is the large amount of available for grain and seed crops are much like those of information and the difficulty of locating the spe- most agricultural crops. Similar maps could be cific material needed by a particular user. In Min- constructed for suitability of land for wetland hab- nesota the most frequent nonfarm users wanted itat for wildfowl, open-land habitat for pheasant soil survey information to determine land values and bobwhite quail, or woodland habitat for for purchase, sale, rent, or as collateral for loans ruffed grouse, squirrel, or deer. by financial institutions. Other nonfarm uses were made by local government officials, zoning ad- ministrators, sanitarians, and tax assessors. Many L C C M persons make frequent use of the Soil Survey Re- The Soil Conservation Service of the U.S. Depart- port. To increase the usefulness and the speed ment of Agriculture developed eight land-use ca- with which information can be extracted from Soil pability classes based on limitations due to ero- Survey Reports, computers are being used to de- sion, wetness, rooting zone, and climate. The velop digitized records (files) of soil maps and limitations and restrictions for these land use ca- soil data bases. Computers help to solve the prob- pability classes are given in Table 19.2. Classes I lem of locating information by locating only the through IV apply to land suited for agriculture and particular part of the record that the user needs. classes V through VIII to land generally not suited Another big advantage of computerized soil sur- to cultivation. Class I land has few use restrictions vey informtion is that the soils' data bases can be

324 SOIL SURVEYS AND LAND-USE I NTERPRETATIONS

TABLE 19.2 Land Capabilty Classes

Land Sied o Cliaion and Ohe Ue Class 1. Soils have few limitations that restrict their uses. Class Il. Soils have some limitations that reduce the choice of plants or require moderate conservation practices. Class IIl. Soils have severe limitations that reduce the choice of plants, require special conservation practices, or both. Class IV. Soils have very severe limitations that restrict the choice of plants, require very careful management, or both. Land Limied in Ue-Geneall No Sied fo Cliaion Class V. Soils have little or no erosion hazard, but have other limitations impractical to remove that limit their use largely to pasture, range, woodland, or wildlife food and cover. Class VI. Soils have severe limitations that make them generally unsuited to cultivation and limit their use largely to pasture or range, woodland, or wildlife food and cover. Class VII. Soils have very severe limitations that make them unsuited to cultivation FIGURE 19.8 Land capability map for a 200-acre and that restrict their use largely to dairy farm in Wisconsin. Roman numerals designate grazing, woodland, or wildlife. Class VIII. Soils and landforms have limitations land capability classes; other symbols indicate land characteristics. For example, 30E37, 30 designates that preclude their use for type of soil, E indicates slope, and 37 means that commercial plant production and more than 75 percent of the topsoil has been lost restrict their use to recreation, and that there are occasional gullies. (Courtesy wildlife, or water supply, or to USDA, Soil conservation Service.) esthetic purposes.

combined with systems to produce a Geographic SOIL SURVEYS AND Information System (GIS). Within these computer AGROTECHNOLGY TRANSFER files information is stored, such as land own- ership, elevation-topographic information, vege- Field experiments are conducted to test hypothe- tation, and land use. Systems are being developed ses for increasing the amount and efficiency of cooperatively with university and government per- agricultural production. Suppose an experiment sonnel so that natural resources (soil, water, wet- indicated that a 3-ton application of lime was the land, etc.) can be better protected through im- most profitable rate of application for production proved land-use planning. of alfalfa in a particular crop rotation system. The REFERENCES 325 question that arises is: Where can this informa- persons, including farmers, governmental offi- tion be used or applied? If the experiment was cals, land appraisers and tax assessors, land-use conducted in the state of Arkansas, where in Ar- and zoning personnel, and ordinary citizens. kansas are the results valid? Can the experimental Soil survey information and interpretations are results be used outside of Arkansas, and if so, being computerized to facilitate access by users where? and to allow for better planning by various govern- The transfer of this kind of information is valid mental agencies. only for locations with very similar soils, climate, Because research information can only be ob- and soil management practices. Thus, it is very tained on a limited number of soils, soils that are important to (1) establish field experiments on expected to perform similarly must be grouped the kinds of soils or mapping units for which together for making interpretations. interpretations or recommendations are needed, and (2) restrict the recommendations or predictions to those soils or very similar soils and experimental conditions. Because experiments cannot be conducted for every mapping unit or REFERENCES soil, it becomes necessary to extrapolate and in- Allan, P. F., L. E. Garland, and R. F. Dugan. 1963. "Rat- terpret the information for situations other than ing Northeastern Soils for Their Suitability for Wildlife those represented by the experiments. Habitat." Trans. 28th Nor. Am. Wildlife and Nat. Res. Con. Anderson, J. 1981. "A News Letter for Minnesota Coop- erative Soil Survey." Soil Survey Scene 3, (4):1-3. SUMMARY University of Minnesota, St. Paul. Bartelli, L. J., A. A. Klingbiel, J. V. Baird, and M.K.R. A soil survey is a systematic examination and Heddleson. 1966. Soil Surveys and Land Use Plan- mapping of the soils of an area. The soil map, ning. Soil Sci. Soc. Am., Madison, Wis. together with information about the soils and Soil Conservation Service, 1961. Land-Capability Classi- land-use interpretations, is published as a Soil fication. USDA, Washington, D.C. Survey Report. Soil Conservation Service. 1981. Soil Survey Manual. Soil Survey Reports are used by a wide variety of 430 V-SSM, USDA, Washington, D.C. CHAPTER 20

LAND AND THE WORLD FOOD SUPPLY

Since the beginning of time, increased food pro- places in the world. It is uncertain why early peo- duction was mainly the result of using more land. ple gave up the more leisurely hunting and gather- Now, new cropland supplies are diminishing as ing activities and took up a more rigorous way of the human population continues to increase. This living to produce crops and animals. Develop- chapter considers the adequacy of the world's ment of agriculture resulted in both an increased land, or soil, resources for the future. food supply and a population explosion. After reaching a population of 5 million over several million years, the human population increased 16 times to 86 million within the next 4,000 years POPULATION AND FOOD TRENDS (10,000 to 6,000 years before present). Population Human population growth and food supply are growth tended to level off, and the world popula- ancient problems. For several million years, hu- tion increased only six to seven times to 545 mil- mans migrated onto new lands and slowly im- lion over the next 6,000 years. This was followed proved their hunting and gathering techniques. by the Industrial Revolution that began about The population increased slowly; about 10,000 1650. years ago there were an estimated 5 million people in the world. At this time, people had just begun to colonize the last remaining continents, T I R the Americas, by crossing over the land bridge The Industrial Revolution brought energy and ma- between Asia and Alaska during the last ice age. chines into the food production process, resulting in an increased food supply. However, a new di- mension was added to population growth-a re- D A duced death rate because of the development of Settled agriculture was just getting started in the medicine and public sanitation. Now, the world Fertile Crescent of the Middle East when the mi- population increases annually at the rate of 1.7 gration to the Americas began. Agriculture may percent and doubles every 40 years. The most have developed at the same time in other isolated rapid rate of increase is in Africa, where the an-

326 POPULATION AND FOOD TRENDS 327 nual increase is 2.9 percent, with a doubling every R T F P 24 years. Western Europe has a growth rate of 0.2 As recently as the period from 1950 to 1975, there percent and a population doubling about every was a 98 percent increase in world cereal produc- 400 years. The slowest population growth is in tion and a 22 percent increase in cropland. In Europe, where some countries have negative pop- recent decades, during a period of rapid popula- ulation growth rates. tion growth, world food production was able to Estimates of world population exceeding 6 bil- keep pace with population growth. In fact, the rate lion by the year 2000 will require about 40 percent of food production has exceeded the population more food just to maintain the status quo. Con- growth rate. For the period from 1971 to 1980, sidering that more than 500 million people are both the developing and developed countries in- currently suffering from malnutrition, and increased food production faster than population, creased food consumption rates are occurring in as shown in Figure 20.1. For Africa, however, pop- the richest parts of the world, it appears that food ulation increased faster than food production, needs, shortly beyond the year 2000, will be dou- which caused a 14 percent decrease in per capita ble today's needs. It is predicted that there will be food production during the 1970's. a world population of 12 billion or more by the There has been a declining per capita grain twenty-second century. production trend for the past 20 years in Africa 328 LAND AND THE WORLD FOOD SUPPLY

and in the Andean countries of Bolivia, Chile, the Amazon Basin and in the central uplands of Ecuador, and Peru. Although total world food pro- Brazil and in the outer islands of Indonesia. Con- duction is increasing, the rate of increase is slow- versely, the area of cropland peaked in the 1950s ing. Since the late 1970s, there appears to be a and 1960s in many Western nations, with declines declining trend in world per capita food produc- from a maximum in 1980 of 29.4 percent for Ire- tion and, now, 50 to 60 nations have declining per land, 21 percent for Sweden, and 19.6 percent for capita food production, as shown in Figure 20.2. Japan. About 2 billion people will be added to the world in another two or three decades, which R T P C C means that a great deal of land will be needed for Expansion onto new lands has been the tradi- nonagricultural use. When new families are tional means to increase the food supply. One formed in many developing nations, houses are reason for the present declining world per capita built on cropland with fields immediately adja- food production is the slower rate of bringing new cent to them. The use of local building materials land into production. The rate of cropland in- (such as bamboo) make multistoried housing im- crease has declined from an annual increase of practical. Much land will also be used for roads, 1.0 percent in the 1950s to 0.3 percent in the strip mining, and transportation facilities, as 1970s, and is expected to be 0.15 percent in the shown in Figure 20.3. 1990s. New settlement projects are underway in At the 1978 International Soil Science Congress POTENTIALLY AVAILABLE LAND AND SOIL RESOURCES 329

FIGURE 20.3 Farmland in the United States is converted to urban use at the annual rate of about 400,000 hectares (1 million acres). held in Edmonton, Canada, concern was ex- forced onto lands with greater production hazards pressed about Canada's ability to remain self- and limitations as shown in Figure 20.4. sufficient in food supply by the year 2000 and beyond. A population growth of 20 to 45 percent is expected, and cities continue to expand onto S S prime farmland. The amount of land for cereal The past few decades have brought an end to the grains per capita was predicted to decline from traditional expansion onto new lands as a major 0.184 hectares in 1978 to 0.128 hectares by 2000. means to increase food production. For the fore- Only three of Canada's Prairie provinces are sur- seeable future, it appears that an expanding world plus food producers. Only 10 percent of the food population will have to produce its food on a produced is now exported. declining amount of land per person. Land requires considerable inputs to remain productive, and it is subject to degradation by erosion, salinity, water-logging, and deserti- POTENTIALLY AVAILABLE LAND AND fication. A recent United Nations report estimated SOIL RESOURCES that 10 percent of the world's irrigated land has About 65 percent of the ice-free land has a climate had a significant decline in productivity due to suitable for some cropping. Only 10 or 11 percent water-logging, and another 10 percent due to an of the world's land is cultivated, which suggests a increase in salinity. In some irrigated regions, considerable possibility for increasing the world's geologic groundwater will be exhausted. In other cropland. cases, the high cost of pumping water and competition with urban water users will result in conversion of irrigated land to range land. Deserti- W P A L fication is a problem, especially in subSaharan The distribution of the world's cropland is shown Africa. As the need for land grows, farmers are in Figure 20.5. The five countries with the most FIGURE 20.4 Farmers are pushed higher and higher on slopes in the Himalayan foothills due to population growth. Many such lands are used only a few years before it is no longer profitable to crop the land due to severe soil erosion and landslides. POTENTIALLY AVAILABLE LAND AND SOIL RESOURCES 331 cropland, in decreasing order, are: USSR, United subtropical areas of Africa and South America. States, India, China, and Canada. The cropland China and India are the two largest and most tends to exist either north or south of a large populated countries in Asia. At present in Asia, central band where there is only a small percent- there is only 0.7 acre of cultivated land per person, age of cropland, in northern South America and and little additional unused and potentially arable Africa and through central Asia. Much of the land exists. India is unique as a large country, cropland in the USSR, United States, and Canada since it has one of the the greatest agricultural is in areas where the dominant soils are Mollisols. potentials of any country, because of its soils and Few new cropland areas are available in western climate. This is reflected in the fact that about 50 Europe and India, as can be seen by examining percent of India's land is currently cultivated, as Figure 20.5. compared to only 10 percent for the world and 20 Many estimates of potential cropland, or arable percent for the United States. land, have been made. Several well-documented The developing countries, with 77 percent of studies concluded that the present cultivated land the world's population, have 54 percent of the area could at least be doubled. The data from one current cultivated land area and 78 percent of the of the studies, (see Table 20.1) show the poten- undeveloped, potentially arable land. By contrast, tially arable land and currently cultivated land by the developed countries have 23 percent of the continents. The data support the following con- world's population, 46 percent of the cultivated clusions: land, and 28 percent of the undeveloped and potentially arable land. Currently, the developed 1. The world's arable land can be increased 100 countries have about twice as much cultivated percent or more. land on a per capita basis than do the developing 2. Of the potentially arable land that can be de- nations. Note that Europe has about as much cul- veloped, 66 percent is located in Africa and tivated land per capita as does Asia, which has South America. essentially no unused, potentially arable land. In The 66 percent potentially arable land that can fact, the actual cropland area in many European be developed is located mainly in tropical and countries is declining. 332 LAND AND THE WORLD FOOD SUPPLY

L W S R S S On a global basis, the main limitations for using The world's arable (or cultivated) land can be the world's soil resources for agricultural produc- doubled with reasonable inputs. Increasing food tion are drought (28%), mineral stress or infertility production by bringing more land into cultivation (23%), shallow depth (22%), excess water (10%), in South America is limited mainly by soil inferti- and permafrost (6%). Only 11 percent of the lity, but also by available water or drought. In world's soils are without serious limitations, as Africa, the major limitations to bringing new land shown in Table 20.2. Because the currently used into cultivation are first, lack of water or drought agricultural land represents the world's best land, and, second, soil infertility. development of the unused, potentially arable land would be expected to have more serious FUTURE OUTLOOK limitations than those shown in Table 20.2. Future improvements in the world food situa- In New York state, wheat yields increased only 3.1 tion, in terms of new land for cultivation, will bushels per acre (209 kg per hectare) over the require developing land in Africa and South 70-year period from 1865 to 1935. This also ty- America. Almost 50 percent of the South Ameri- pifies the nature of crop yield increases in Europe can continent is centered on the Amazon Basin during the same period. Beginning in 1935, im- and central uplands of Brazil, where soils are proved varieties and better management practices mainly Oxisols and Ultisols. Not surprisingly, 47 increased wheat yields 20.1 bushels per acre dur- percent of soil-use limitations are related to mining the next 40 years, as shown in Figure 20.6. eral stress and only 17 percent are due to drought. Fifty-one percent of the increase was due to new In Africa, 44 percent of the area has an arid or technology and 49 percent of the increase was semiarid climate. Many Oxisols and Ultisols oc- due to improved varieties of wheat. Low energy cupy the Zaire (Congo) basin, but an enormous costs coupled with the adoption of improved vari- area of Ustalfs occurs along the subSaharan re- eties and other technology have resulted in a con- gion. This reflects the 44 percent drought and only tinuing overproduction of food in the United 18 percent mineral stress limitations in Africa. States.

TABLE 20.2 World Soil Resources and their Major Limitations for Agricultural Use

From The State of Food and Agriculture 1977, FAO, 1978. 1 Nutritional deficiencies or toxicities related to chemical composition or mode of origin. FUTURE OUTLOOK 333

FIGURE 20.6 Wheat yields in New York State from 1866- 1975. Between 1935 and 1975, yield increase was the result A 51 percent to technology and 49 percent to genetic gain. (Adapted from Jensen, 1978, and used by permission of Science.)

Large yield increases are common when the The World Grain Trade base yield is very low, which accounts for much of The extent of food production sufficiency in vari- the relatively large increase in food production in ous parts of the world can be deduced in part from many developing nations in recent years (see Fig- an observation of the world grain trade. Western ure 20.1). In the future, yield increases will be- Europe, after the Industrial Revolution, became come more difficult as the yield base increases. dependent on other countries for food grains and Thus, with very little new land coming into use remained a consistent importer for a very long and the need to increase yields on current period of time. We have noted that Europe has cropland, increases in food production are be- little more cropland per capita than does Asia, coming more difficult. and Europe has little unused potentially arable land to bring into production. The large and consistent import of grain by Europe since the 1930s Beyond Technology is shown in Table 20.3. From a technological point of view, a large poten- Many nations that are now importers of food tial for increasing food production could be real- grains were exporters during the 1930s (see Table ized by greater photosynthetic efficiency, greater 20.3). India was a wheat exporter until population use of biological nitrogen fixation, more efficient growth overtook food production growth. Latin water and nutrient use by plants, and greater plant America and even Africa were food grain export- resistance to disease and environmental stress. In ers. The USSR was an exporter before the 1917 many developing countries, technology has Russian revolution. Its inability to provide ade- bought about large and rapid increases in food quate diets, and the resulting riots, caused the production, only to be overwhelmed by popula- USSR to enter the world market and buy a large tion growth. Without a suitable social, economic, amount of grain in 1973. This produced the so- and political environment, the benefits of technol- called great grain robbery as the grain was se- ogy have, in some cases, been temporal. cretly purchased without allowing the increased 334 LAND AND THE WORLD FOOD SUPPLY

demand to affect prices. Note from Table 20.3 that North America-the United States and the USSR is a major grain importer in spite of a Canada-together with Argentina, Australia, and near stagnant rate of population growth. The New Zealand, have become the world's exporters USSR has more cultivated land per person than of food grains. The data in Table 20.3 show that does North America (see Table 20.1). Production despite great efforts to increase food production problems stem from their unique economic- in the developing countries, the food balance situ- political situation and less than ideal climatic ation has deteriorated, a trend that is expected to conditions. The large imports of grain into the continue. USSR from the United States, which began in the early 1970s, are continuing. Much of the imported grain is fed to farm animals (see feed grains in P C P Figure 20.7). A population growth rate of zero is a mathemati- cal certainty. Even a low growth rate, over a long FIGURE 20.7 Grain-exports to the Soviet Union from enough period of time, results in more people the United States. The increase in exports has than there is standing room available on earth. At continued thorough the 1980s. (Data USDA.) the extreme of estimates for the earth's carrying capacity, deWit (1967) calculated that if all of the land surface produced calories equivalent to the maximum photosynthetic potential, there would be enough calories for 1,000 billion people. Such a world would have no place for people to live and work and no room for animals. Population control as government policy is difficult because of long-time family traditions, religious beliefs, and a diminished sense of importance as the country's population declines. A one-child family policy was adopted by China to curb population growth. Today, the slowest rates of population growth are in the areas with the greatest amount of resources to invest in food REFERENCES 335 production. Population control programs warrant ical problems are solved. Today, the statement is more support. still true. Looking at the food-population problem In Africa, south of the Sahara, some countries from a technological point of view, one can be are contributing to a food reserve while other optimistic. Looking at the problem in its totality, it countries are food deficient. Drought has been is hard to be optimistic. The difficult task ahead is used to explain the food shortage in some of these to provide the economic, social, and political en- African countries. It is becoming more and more vironment in which hundreds of millions of small apparent, however, that a lack of appropriate in- peasant farmers, and with simple tools, can pro- frastructures is a problem. In Zambia, there are duce enough food for their families and some few people within a large area. In spite of low surplus to sell to the urban sector. The problem is population, there has been considerable erosion an ancient one, and the challenge for its solution and degradation of the most fertile soils. Over- continues. grazing also contributes to soil erosion and land degradation. Yet, in Zambia there is no soil conservation service and little effort has been made at the national level to control land use. In Nepal, population growth has forced farmers REFERENCES to plant crops on higher and steeper land in the Bentley, C. F. 1978. "Canada's Agricultural Land Re- hills. The result is extensive soil erosion and even- sources and the World Food Problem," in Trans. 11th tual abandonment of the land as a result of land- Int. Cong. Soil Sci. 2:1-26, Edmonton, 1978. slides. Erosion is further promoted by the removal Blaikie, Piers. 1985. The Political Economy of Soil Ero- of the forest cover for fuel wood, which has re- sion in Developing Countries. Longman, London. sulted in severe flooding downstream. Depletion Brown, H. 1954. The Challenge of Man's Future. Viking, of the forest (fuel wood) and subsequent conver- New York. sion to kerosene to cook food may absorb 15 to 20 Brown, L. R. 1978. "Worldwide Loss of Cropland." percent of a poor family's income. Erosion is an Worldwatch Paper 24, Worldwatch Institute, Wash- ington, D.C. ever present and pressing problem for small farm- Brown, L. R. 1985. "Reducing Hunger." State of the ers and results in higher fuel costs for the urban World, pp. 23-41, Norton, New York. poor. The political power, however, rests with Brown, L. R. 1989. State of the World. Norton, New York. those who are not directly affected. Some of these deWit, C. T. 1967. "Photosynthesis: Its Relationship to include wealthy people who own rental properties Overpopulation." Harvesting the Sun, A. J. Pietro, in the capital city, the large landowners with es- F. A Greer, and T. J. Army, Eds, pp. 315-332, Aca- tates on the fertile strip of land along the Indian demic, New York. border (the Terai), and merchants who import FAO. 1978. The State of Food and Agriculture 1977. goods for resale. These groups wield the most Rome. political power in Nepal, so the needs of the urban FAO. 1981. The State of Food and Agriculture 1980. Rome. and rural poor are ignored. Foth, H. D. 1982. Soil Resources and Food: A Global View, in Principles and Applications of Soil Geogra- phy. E. M. Bridges and D. A. Davidson, eds., Long- SUMMARY man, London. Jensen, N. F. 1978. "Limits to Growth in World Food In 1948, the late Dr. Charles E. Kellogg pointed out Production." Science. 210:317-320. that sufficient land and soil resources (and other Kellogg, C. E. 1948. "Modern Soil Science." Am. Scien- physical resources) are available to feed the tist. 38:517-536. world, provided, that economic, social, and polit- Kellogg, C. E. and A. C. Orvedal. 1969. "Potentially 336 LAND AND THE WORLD FOOD SUPPLY

Arable Soils of the World and Critical Measures for The White House. 1967. The World Food Problem. U.S. Their Use," in Advances in Agronomy. Vol. 21. Aca- Government Printing Office, Washington, D.C. demic Press, New York. USDA. 1964. "A Graphic Summary of World Agricul- Morgan, Dan. 1979. Merchants of Grain. Viking, New ture." Misc. Pub. No. 705. Washington, D.C. York. APPENDIX I

SOIL TEXTURE BY THE FIELD METHOD

337 338 SOIL TEXTURE BY THE FIELD METHOD APPENDIX II

TYPES AND CLASSES OF SOIL STRUCTURE 340 APPENDIX III

PREFIXES AND THEIR CONNOTATIONS FOR NAMES OF GREAT GROUPS IN THE U.S. SOIL CLASSIFICATION SYSTEM (SOIL TAXONOMY)

341 GLOSSARY

A . Mineral horizons that formed at the sur- A . The state of dryness at equilibrium with the face, or below an 0 horizon, and are characterized by water content in the surrounding atmosphere. an accumulation of humified organic matter intimately A . A mineral soil horizon from which mixed with the mineral fraction. clay and free iron oxides have been removed or segre- A . Soil with a pH value < 7.0. gated. The color of the horizon is determined primarily A . Hydrogen ions or cations that, on by the color of primary sand and silt particles rather than coatings on these particles. An E horizon. being added to water, undergo hydrolysis,H+, resulting in an acidic solution. Examples in soils are A1 3+ , and Alfisols. Mineral soils that have umbric or ochric epi- Fe 3+ pedons, agrillic or kandic horizons, and that have plant- A, . Soil acidity that is neutralized by available water during at least 90 days when the soil is lime or other alkaline materials, but that cannot be warm enough for plants to grow. Alfisols have a mean replaced by an unbuffered salt solution. annual soil temperature below 8° C or a base saturation in the lower part of the argillic horizon of 35 percent or A, -. The aluminum and hy- more when measured at pH 8.2. A soil order. drogen that can be replaced from an acid soil by an unbuffered salt solution such as KCI. Essentially, the A . Any soil having a pH above 7.0. sum of the exchangeable Al and H. Ammonification. The biological process leading to A, . Sum of salt-replaceable and resid- the formation of ammoniacal nitrogen from nitrogen- ual acidity. containing organic compounds. Actinomycetes. A nontaxonomic term applied to a A . The process of converting ex- group of gram-positive bacteria that have a superficial changeable or soluble ammonium ions to those occu- + resemblance to fungi. pying interlayer positions similar to K in micas. Activity. Informally, may be taken as the effective A . Noncrystalline soil consti- concentration of a substance in a solution. tutents. A. To allow or promote exchange of soil gases A. (1) The absence of molecular oxygen. with atmospheric gases. (2) Growing in the absence of molecular oxygen, such as anaerobic bacteria. (3) Occurring in the absence of A . The fraction of the bulk soil vol- oxygen, such as a biochemical process. ume that is filled with air at any given time, or under a given condition, such as a specified soil-water matric Andisols. Mineral soils developed in volcanic ejecta potential. that have andic soil properties. A soil order. A. (1) Having molecular oxygen as apart of the A . The sum total of ex- environment. (2) Growing only in the presence of mo- changeable anions that a soil can adsorb. lecular oxygen, as aerobic organisms. (3) Occurring Aquic. A mostly reducing soil moisture regime nearly only in the presence of molecular oxygen, such as free of dissolved oxygen due to saturation by ground- aerobic decomposition. water or its capillary fringe and occurring at periods Aggregate. A unit of soil structure, usually formed when the soil temperature at 50 centimeters below the by natural processes, and generally below 10 milli- surface is above 5° C. meters in diameter. A . A mineral soil horizon that is characterized by the illuvial accumulation of layer-silicate * Adapted from Glossary of Soil Science Terms, published by clays. the Soil Science Society of America, July 1987, and reprinted Aridic. A soil moisture regime that has no plant- by permission of the Soil Science Society of America. available water for more than half the cumulative time

342 GLOSSARY 343

3 that the soil temperature at 50 centimeters below the g/cm . The bulk volume is determined before drying to surface is above 5° C and has no period as long as 90 constant weight at 105° C. consecutive days when there is water for plants, while the soil temperature at 50 centimeters is continuously C . Horizons or layers, excluding hard rock, above 8° C. that are little affected by the soil-forming processes. C horizons typically underly A, B, E, or 0 horizons. Aridisols. Mineral soils that have an aridic moisture regime but no oxic horizon. A soil order. C . Soil containing sufficient free Ca- C0 3 , and/or MgC0 3 , to effervesce visibly when treated A. An organism capable of utilizing C0 2 or carbonates as a sole source of carbon and obtaining with cold 0.1 molar HCI. These soils usually contain energy for carbon reduction and biosynthetic processes from as little as I to 20 percent CaC0 3 equivalent. from radiant energy (photoautotroph) or oxidation of C . A mineral soil horizon of secondary inorganic substances (chemoautotroph). carbonate enrichment that is more than 15 centimeters thick, has a CaC0 equivalent above 150 g/kg, and has A . Nutrient ions or compounds in 3 at least 50 g/kg more CaC0 equivalent than the under- forms that plants can absorb and utilize in growth. 3 lying C horizon. A . The portion of water in a soil that can be absorbed by plant roots; the amount of water Caliche. A zone near the surface, more or less ce- between in situ field capacity and the wilting point. mented by secondary carbonates of calcium or magnesium precipitated from the soil solution. Caliche may occur as a soft thin soil horizon, as a hard thick bed, or B horizons. Horizons that formed below an A, E, or 0 as a surface layer exposed by erosion. horizon that have properties different from the overlying C . A mineral soil horizon that has a and underlying horizons owing to the soil forming texture of loamy very fine sand or finer, has soil struc- processes. ture rather than rock structure, contains some weather- Bacteroid. An altered form of cells of certain bac- able minerals, and is characterized by alteration or teria. Refers particularly to the swollen, irregular vacu- removal of mineral material, or the removal of carbo- olated cells of Rhizobium in nodules of legumes. nates. Bar. A unit of pressure equal to 1 million dynes per C . A zone in the soil just above the square centimeter. water table that remains saturated or almost saturated B e. The extent to with water. which the cation exchange capacity is saturated with C- . The ratio of the alkali (sodium and potassium) and alkaline earth (cal- mass of organic carbon to the mass of organic nitrogen cium and magnesium) cations expressed as a percent- in soil, organic material, plants, or the cells of microor- age of the cation exchange capacity, as measured at a ganisms. particular pH, such as 7 or 8.2. C . Wet clay soils containing ferrous sulfide, Biomass. The total mass of living microorganisms in which become highly acidic when drained. a given volume or mass of soil. Catena. A sequence of soils of about the same age, Biosequence. A sequence of related soils, differing derived from similar parent material, and occurring one from the other, primarily because of differences in under similar climatic conditions, but having different kinds and numbers of plants and soil organisms as a characteristics because of variations in relief and in soil-forming factor. drainage. Biuret. A toxic product formed at high temperature C . The interchange between a cation during the manufacturing of urea. in solution and another cation on the surface of any B . The ability of ions associated with the negatively charged material such as clay colloid or solid phase to buffer changes in ion concentration in organic colloid. the solution phase. C (CEC). The sum of ex- Bulk density, soil. The mass of dry soil per unit bulk changeable cations that a soil, soil constitutent, or volume, expressed as grams per cubic centimeter, other material can adsorb at a specific pH; commonly 344 GLOSSARY expressed as milliequivalents per 100 grams or cen- adjacent soil or snow, or from intercepted precipitation timoles per kilogram. in any specified time. Usually expressed as equivalent C . Informally, it is the capacity of a depth of free water per unit of time. solution or other substance to do work by virtue of its C. Slow mass movement of soil and soil material chemical composition. down relatively steep slopes primarily under the influ- Chlorosis. Failure of plants to develop chlorophyll ence of gravity. caused by a deficiency of an essential element. Chloro- Crust. A soil-surface layer, ranging in thickness from tic leaves range in color from light green through yellow a few millimeters to a few tens of millimeters, which, to almost white. when dry, is much more compact, hard, and brittle than Chroma. The relative purity, strength, or saturation of the material immediately beneath it. a color; directly related to the dominance of the deter- Cryic. A soil temperature regime that has mean an- mining wavelength of the light and inversely related to nual soil temperatures of above 0° C but below 8° C at 50 grayness; one of the three variables of color. centimeters. There is more than a 5° C difference be- Chronosequence. A sequence of related soils that tween mean summer and mean winter soil tempera- differ one from the other, in certain properties primarily tures. as a result of time as a soil-forming factor. C . That part of the total D . A law describing the rate of flow of phosphorus in fertilizer that is insoluble in water but water through porous media. Named for Henry Darcy of soluble in neutral 0.33 M ammonium citrate and which, Paris, who formulated it in 1856 from extensive work on together with water-soluble phosphorus represents the the flow of water through sand filter beds. readily available phosphorus content of the fertilizer. Denitrification. Reduction of nitrate or nitrite to mo- C. (1) A soil separate consisting of particles lecular nitrogen or nitrogen oxides by microbial activity <0.002 millimeters in equivalent diameter. (2) A tex- or by chemical reactions involving nitrite. tural class. D . The layer of gravel or stones re- C . Coatings of clay on the surfaces of soil maining on the land surface in desert regions after the peds, mineral grains, and in soil pores. (Also called removal of the fine material by wind erosion. clay skins, clay flows, or agrillans.) D . A heterogeneous system that C . Any crystalline inorganic substance of consists of a solid surface layer having a net electrical clay size. charge, together with an ionic swarm under the influ- Claypan. A dense, compact layer in the subsoil hav- ence of the solid and a solution phase that is in direct ing a much higher clay content than the overlying mate- contact with the surface. rial, from which it is separated by a sharply defined Duripan. A mineral soil horizon that is cemented by boundary. Claypans usually impede the movement of silica to the point that air-dry fragments will not slake in water and air and the growth of plant roots. water or HCI. Colluvium. A general term applied to deposits on a slope or at the foot of a slope or cliff that were moved E . Mineral horizons in which the main there chiefly by gravity. feature is loss of silicate clay, iron, aluminum, or some Concretion. A local concentration of a chemical combination of these, leaving a concentration of sand compound, such as calcium carbonate or iron oxide, in and silt particles of quartz or other resistant minerals. the form of a grain or nodule of varying size, shape, ECe . The electrolytic conductivity of an extract from hardness, and color. saturated soil, normally expressed in units of deci- Consistency. The manifestations of the forces of co- siemens per meter at 25° C. hesion and adhesion acting within the soil at various Ecology. The science that deals with the interre- water contents, as expressed by the relative ease with lationships between organisms and between organ- which a soil can be deformed or ruptured. isms and their environment. C . The water used by plants Ecosystem. A community of organisms and the envi- in transpiration and growth, plus water vapor loss from ronment in which they live. GLOSSARY 345

Ectomycorrhiza. A mycorrhizal association in which essential for plant growth without a toxic concentration the fungal mycelia extend inward, between root cortical of any element. cells, to form a network ("Hartig net") and outward into Fertilizer. Any organic or inorganic material of natu- the surrounding soil. ral or synthetic origin (other than liming materials) that Edaphology. The science that deals with the influ- is added to a soil to supply one or more elements ence of soils on living things, particularly plants. essential to the growth of plants. Eluviation. The removal of soil material in suspen- F . Mostly undecomposed plant re- sion (or in solution) from a layer or layers of soil. mains that contain large amounts of well-preserved and Endomycorrhiza. A mycorrhizal association with in- recognizable fibers. tracellular penetration of the host root cortical cells by F , ( ). T the fungus as well as outward extension into the sur- content of water, on a mass or volume basis, remaining rounding soil. in a soil 2 or 3 days after having been wetted with water Entisols. Mineral soils that have no distinct sub- and after free drainage is negligible. surface diagnostic horizons within 1 meter of the soil F . A thin layer of water, in close proximity surface. A soil order. to soil-particle surfaces, that varies in thickness from I E. (1) The wearing away of the land surface by or 2 to perhaps 100 or more molecular layers. running water, wind, ice, and other geological agents, Fixation. The process by which available plant nutri- including such processes as gravitational creep. (2) ents are rendered less available or unavailable in the Detachment and movement of soil or rock by water, soil. wind, ice, or gravity. Floodplain. The land bordering a stream, built up of E . A numerical value expressing the sediments from overflow of the stream, and subject to inherent erodibility of a soil. inundation when the stream is at flood stage. Eutrophic. Having concentrations of nutrients opti- Flux. The time rate of transport of a quantity across a mal, or nearly so, for plant or animal growth. given area. Evapotranspiration. The combined loss of water F . A general term for those iron ox- from a given area, and during a specified period of time, ides that can be reduced and dissolved by a dithionite by evaporation from the soil surface and by transpira- treatment. Often includes goethite and hematite. tion from plants. Friable. A consistency term pertaining to the ease of E . The extent to crumbling of soils. which the adsorption complex of a soil is occupied t F . The colored material that remains in so- particular cation. lution after the removal of humic acid by acidification. E . A cation or anion held on or near the surface of a solid particle, which may be replaced by other ions of similar charge that are in solution. Gibbiste. A mineral with a platy habit that occurs in highly weathered soils. Al(OH) 3 . . Gilagi. The microrelief of soils produced by expan- Fallowing. The practice of leaving land uncropped sion and contraction with changes in water content, a for periods of time to accumulate and retain water and common feature of Vertisols. mineralized nutrient elements. G . Rock debris that has been transported F, . In soil classification one of the catego- by glaciers and deposited, either directly from the ice or ries intermediate between the great soil group and the from the meltwater. soil series. Families provide groupings of soils with Goethite. A yellow-brown iron oxide mineral that is ranges in texture, mineralogy, temperature, and very common and is responsible for the brown color in thickness. many soils. FeOOH. Fertigation. Application of plant nutrients in irri- G . One of the categories in the system gation water. of soil classification that has been used in the United F, . The ability of a soil to supply elements States for many years. Great groups place soils accord- 346 GLOSSARY

ing to soil moisture and temperature, basic cation satu- decomposition products, and the soil biomass. Resis- ration status, and expression of soil horizons. tant to further alteration. G . Plant material incorporated into soil H . The proportionality factor while green or at maturity; for soil improvement. in Darcy's Law as applied to the viscous flow of water in Groundwater. That portion of the water below the soil. surface of the ground at a pressure equal to or greater H-. Aluminum hyroxide com- than atmospheric. pounds of varying composition. Guano. The decomposed dried excrement of birds Hyperthermic. A soil temperature regime that has and bats, used for fertilizer purposes. mean annual soil temperatures of 22° C or more and a G . A mineral soil horizon of secondary higher than 5° C difference between mean summer and calcium sulfate enrichment that is more than 15 cen- mean winter soil temperatures at 50 centimeters below timeters thick. the surface. Gypsum. The common name for calcium sulfate (Ca- . S0 4 2H 20), used to supply calcium and sulfur to I . A soil layer or horizon in which ameliorate sodic soils. material carried from the overlying layer has been precipitated from solution or deposited from suspension. Hardpan. A hardened soil layer, in the lower A or in The layer of accumulation. the B horizon, caused by cementation of soil particles Illuviation. The process of deposition of soil material with organic matter or with materials such as silica, removed from one horizon to another in the soil, usu- sesquioxides, or calcium carbonate. ally from an upper to a lower horizon in the soil profile. H . Those metals that have high density; Immobilization. The conversion of an element from in agronomic usuage includes Cu, Fe, Mn, Mo, Co, Zn, the inorganic to the organic form in microbial or plant Cd, Hg, Ni, and Pb. tissues. Hematite. A red iron oxide mineral that contributes Inceptisols. Mineral soils having one or more pedo- red color to many soils. Fe203. genic horizons in which mineral materials, other than H . An intermediate degree of decom- carbonates or amorphorus silica, have been altered or position, such as two-thirds of the organic material removed but not accumulated to a significant degree. cannot be recognized. Water is available to plants more than half of the year or Heterotroph. An organism capable of deriving car- more than 90 consecutive days during a warm season. bon and energy for growth and cell synthesis by the A soil order. utilization of organic compounds. I . Plants characteristic of specific Histosols. Organic soils that have organic soil mate- soil or site conditions. rial in more than half of the upper 80 centimeters, or Infiltration. The downward entry of water through that are of any thickness overlying rock or fragmental the soil surface. materials, which have interstices filled with organic soil I . Informally, the effective concentration materials. A soil order. of an ion in solution. Hue. One of the three variables of color. I . Group name for the oxides and hydrox- H . The dark-colored organic material that ides of iron. Includes the minerals goethite, hematite, can be extracted from soil by various agents and that is lepidocrocite, ferrihydrite, maghemite, and magnetite. precipitated by acidification to pH 1 or 2. Sometimes referred to as sesquioxides or hydrous Humification. The process whereby the carbon of oxides. organic residues is transformed and converted to hu- I . An indurated soil horizon in which iron mic substances through biochemical and/or chemical oxide is the principal cementing agent, as in plinthite or processes. laterite. Humus. All of the organic compounds in soil exclu- Ironstone. Hardened plinthite materials often occur- sive of undecayed plant and animal tissues, their partial ring as nodules and concretions. GLOSSARY 347

I . The replacement of one turally similar to magnetite. Fe 203 . Often found in well- atom by another of similar size in a crystal structure drained, highly weathered soils of the tropical regions. without disrupting or seriously changing the structure. Magnetite. A black, magnetic iron oxide mineral usually inherited from igneous rocks. Often found in Jarosite. A yellow potassium iron sulfate mineral. soils as black magnetic sand grains. M . A group term for oxides of man- K . Subsoil diagnostic horizon having a ganese. They are typically black and frequently occur as clay increase relative to overlying horizons and having nodules and coatings on ped faces, usually in associa- low-activity clays, below 16 meq/100 g or below 16 tion with iron oxides. cmol(+)/kg of clay. Manure. The excreta of animals, with or without an Kaolinite. A clay mineral of the kaolin subgroup. It admixture of bedding or litter, fresh or at various stages has a 1 : 1 layer structure and is a nonexpanding clay of further decomposition or composting. mineral. M (nutrient). The movement of solutes asso- Labile. A substance that is readily transformed by ciated with the net movement of water. microorganisms or is readily available to plants. M . A thick, dark colored surface hori- Landscape. All the natural features such as fields, zon having andic soil properties. hills, forests, water, and such, which distinguish one Mesic. A soil temperature regime that has mean an- part of the earth's surface from another part. Usually nual soil temperatures of 8° C or more but less than 15° that portion of the land that the eye can comprehend in C, and more than 5° C difference between mean sum- a single view. mer and mean winter temperatures at 50 centimeters Lattice. A regular geometric arrangement of points in below the surface. a plane or in space. Lattice is used to represent the M. A layer-structured aluminosilicate mineral distribution of repeating atoms or groups of atoms in a group of the 2: 1 type that is characterized by high layer crystalline substance. charge, which is usually satisfied by potassium. Leaching. The removal of materials in solution from M. The sequence of atmospheric the soil. changes within a very small region. Lepidocrocite. An orange iron oxide mineral that is M. Protozoa, nematodes, and arthropods found in mottles and concretions of wet soils. FeOOH. of microscopic size. L, . A soil amendment containing M. Bacteria (including actinomycetes), calcium carbonate, magnesium carbonate and other fungi, algae, and viruses. materials; used to neutralize soil acidity and furnish Micronutrient. A chemical element necessary for calcium and magnesium for plant growth. plant growth found in small amounts, usually less than Lithosequence. A group of related soils that differ, 100 mg/kg in the plant. These elements consist of B, Cl, one from the other, in certain properties primarily , ; a Cu, Fe, Mn, Mo, and Zn. result of differences in the parent rock as a soil-forming Mineralization. The conversion of an element from factor. an organic form to an inorganic state as a result of L. A soil textural class. microbial activity. Loess. Material transported and deposited by wind M . A soil consisting predominantly of, and and consisting of predominantly silt-sized particles. having properties determined predominantly by, min- L . The absorption by plants of nutrients eral matter. Usually contains less than 200 g/kg of or- in excess of their need for growth. ganic carbon. Mollisols. Mineral soils that have a mollic epipedon Macronutrient. A plant nutrient usually attaining a overlying mineral material with a basic cation satura- concentration of more than 500 mg/kg in mature plants. tion of 50 percent or more when measured at pH 7.0. A Maghemite. A dark, reddish-brown, magnetic iron soil order. oxide mineral chemically similar to hematite, but struc- M. An aluminum silicate (smectite) 348 GLOSSARY with a layer structure composed of two silica tetrahe- since harvest of the previous crop; usually a special dral sheets and a shared aluminum and magnesium planter is necessary to prepare a narrow, shallow seed- octahedral sheet. bed immediately surrounding the seed being planted. Mor. A type of forest humus in which the Oa horizon N . The depressing effect caused is present and in which there is almost no mixing of by one or more plant nutrients on the uptake and avail- surface organic matter with mineral soil. ability of another. M . A layer that is marked with spots or N . A statistical term used when blotches of different color or shades of color (mottles). two or more nutrients are applied together to denote a M . An organic soil in which the plant resi- departure from additive responses occurring when they dues have been altered beyond recognition. are applied separately. Mulch. Any material such as straw, sawdust, leaves, O . plastic film, and loose soil, that is spread upon the soil Layers dominated by organic material, except limnic layers that are organic. surface to protect soil and plant roots from the effects of raindrops, soil crusting, freezing, evaporation, and O . A thin, light colored surface hori- such. zon of mineral soil. M . A system of tillage and planting op- O . A soil that contains a high percentage erations resulting in minimum incorporation of plant of organic carbon (>200 g/kg or >120 - 180 g/kg if residues or other mulch into the soil surface. saturated with water) throughout the solum. Mull. A type of forest humus in which the Oe horizon Outwash. Stratified glacial drift deposited by melt- may or may not be present and in which there is no Oa water streams beyond active glacier ice. horizon. The A horizon consists of an intimate mixture O . Soil that has been dried at 105° C until of organic matter and mineral soil with gradual transi- it reaches constant mass. tion between the A horizon and the horizon under- Oxisols. Mineral soils that have an oxic horizon neath. within 2 meters of the surface or plinthite as a continu- M . A color designation system ous phase within 30 centimeters of the surface, and that that specifies the relative degrees of the three simple do not have a spodic or argillic horizon above the oxic variables of color: hue, value, and chroma. For exam- horizon. A soil order. ple: 10YR 6/4 is a color with a hue = 10YR (yellow - red), value = 6, and chroma = 4. P, . A soil formed on a landscape dur- Mycorrhiza. Literally "fungus root." The association, ing the geological past and subsequently buried by usually symbiotic, of specific fungi with the roots of sedimentation. higher plants. Pans. Horizons or layers in soils that are strongly compacted, indurated, or having very high clay content. N . The relationship between the percentage of P . Similar to lithic contact except water under field conditions and the percentages of that it is softer, and is difficult to dig with a spade. inorganic clay and of humus. P . The unconsolidated and more or N . A mineral soil horizon that satisfies less chemically weathered mineral or organic matter the requirements of an argillic horizon, but that also has from which the solum of soils is developed by pedoge- prismatic, columnar, or blocky structure and a subhori- nic processes. zon having 15 percent or more saturation with ex- P . The density of soil particles, the dry changeable sodium. mass of the particles being divided by the solid volume Nitrification. Biological oxidation of ammonium to of the particles. nitrite and nitrate, or a biologically induced increase in Pascal. A unit of pressure equal to 1 Newton per the oxidation state of nitrogen. square meter. Nitrogenase. The specific enzyme required for bio- Peat. Unconsolidated soil material consisting largely logical dinitrogen fixation. of undecomposed, or only slightly decomposed, or- N- . A procedure whereby a crop is ganic matter accumulated under conditions of exces- planted directly into the soil with no preparatory tillage sive moisture. GLOSSARY 349

Ped. A unit of soil structure such as an aggregate, P2 0 5 . Phosphorus pentoxide; fertilizer label designa- crumb, prism, block, or granule, formed by natural tion that denotes the percentage of available phos- processes. phate. Pedon. A three-dimensional body of soil with lateral P . A black to dark-reddish mineral soil dimensions large enough to permit the study of horizon horizon that is usually thin, is commonly cemented shapes and relations. Its area ranges from 1 to 10 with iron, and is slowly permeable or impenetrable to square meters. water and roots. Penetrability. The ease with which a probe can be P . A soil capable of being molded or de- pushed into the soil. formed continuously and permanently, by relatively Percolation. The downward movement of water moderate pressure, into various shapes. See Consis- through the soil. tency. Pergelic. A soil temperature regime that has mean Plinthite. A nonindurated mixture of iron and alumi- annual soil temperatures of less than 0° C. Permafrost is num oxides, clay, quartz, and other diluents that com- present. monly occurs as red soil mottles usually arranged in platy, polygonal, or reticulate patterns. Plinthite Permafrost. A perennially frozen soil horizon. changes irreversibly to ironstone hardpans, or irregular P . The upper boundary of the perma- aggregates on exposure to repeated wetting and drying. frost, coincident with the lower limit of seasonal thaw. P . An induced subsurface soil horizon or P . The largest water content layer having a higher bulk density and lower total po- of a soil at which indicator plants, growing in that soil, rosity than the soil material directly above and below, wilt and fail to recover when placed in a humid cham- but similar in particle size analysis and chemical ber. Often estimated by the water content at -15 bars, properties. The pan is usually found just below the -1,500 kilopascals, or -1.5 megapascals soil matric maximum depth of primary tillage and frequently re- potential. stricts root development and water movement. Also P . A continuous, indurated calcic called a pressure pan or plow sole. horizon that is cemented by calcium carbonate and, in Polypedon. A group of contiguous similar pedons. some places, with magnesium carbonate. Potash. Term used to refer to potassium or po- P . A continuous, strongly ce- tassium fertilizers and usually designated as K 0. mented, massive gypsic horizon that is cemented with 2 P . The process of converting ex- calcium sulfate. changeable or water-soluble potassium to that occupy- H- . The portion of the cation or + ing the position of K in the micas. anion exchange capacity which varies with pH. P . A mineral that has not been altered H, . The negative logarithm of the hydrogen ion activity of a soil. chemically since deposition and crystallization from molten lava. Phase. A utilitarian grouping of soils defined by soil P, . A vertical section of soil through all its or environmental features that are not class differentia horizons and extending into the * C horizon. used in the U.S. system of soil taxonomy, for example, surface texture, surficial rock fragments, salinity, erosion, thickness, and such. R . Hard bedrock including granite, basalt, Phosphate. In fertilizer trade terminology, phos- quartzite, and indurated limestone or sandstone that is phate is used to express the sum of the water- sufficiently coherent to make hand digging impractical. soluble and citrate-soluble phosphoric acid (P2 05); also referred to as the available phosphoric acid R . A measure of the erosive potential of a specific rainfall event. (P205) P . In commerical fertilizer manufac- R, . The degree of acidity or alkalinity of a turing, it is used to designate orthophosphoric acid, soil, usually expressed as a pH value. H3P04. In fertilizer labeling, it is the common term used Regolith. The unconsolidated mantle of weathered to represent the phosphate content in terms of available rock and soil material on the earth's surface; loose phosphorus, expressed as percent P 2 0 5 . earth materials above solid rock. 350 GLOSSARY

R . The available nutrient content of a S- . A soil in which the surface layer soil carried to subsequent crops. becomes so well aggregated that it does not crust and R . A network of streaks of differ- seal under the impact of rain, but instead serves as a ent color, most commonly found in the deeper profiles surface mulch upon drying. of soil containing plinthite. S, . See Soil series. Rhizobia. Bacteria capable of living symbiotically in Sesquioxides. A general term for oxides and hydrox- roots of leguminous plants, from which they receive ides of iron and aluminum. energy and often utilize molecular nitrogen. S. A nonporphyrin metabolite secreted Rhizosphere. The zone immediately adjacent to by certain microorganisms and plant roots that forms a plant roots in which the kinds, numbers, or activities of highly stable coordination compound with iron. microorganisms differ from that of the bulk soil. S- . The molecules of silicon diox- Rill. A small, intermittent watercourse with steep ide per molecule of aluminum oxide in clay minerals or sides; usually only several centimeters deep and, thus, in soils. no obstacle to tillage operations. S. (1) A soil separate consisting of particles be- Runoff. That portion of the precipitation on an area tween 0.05 and 0.002 millimeters in equivalent diame- which is discharged from the area through stream ter. (2) A soil textural class. channels. S . A quantitative evaluation of the productivity of a soil for forest growth under the existing or S . A nonsodic soil containing sufficient sol- specified environment. Commonly, the height in meters uble salt to adversely affect the growth of most crop of the dominant forest vegetation taken or calculated to plants. an age index such as 25, 50, or 100 years. S . A mineral soil horizon of enrichment Slickensides. Polished and grooved surfaces pro- with secondary salts more soluble in cold water than duced by one mass sliding past another; common in gypsum. Vertisols. S . Intermittent or continuous saline water Smectite. A group of 2:1 layer structured silicates discharge at or near the soil surface under dry-land with high cation exchange capacity and variable inter- conditions, which reduces or eliminates crop growth. layer spacing. S- . A soil containing both sufficient S . A nonsaline soil containing sufficient ex- soluble salt and exchangeable sodium to adversely af- changeable sodium to adversely affect crop production fect crop production under most soil and crop condi- and soil structure under most conditions of soil and tions. plant type. S . The quantity of soluble salt removed S (SAR). A relation between from an irrigated area in the drainage water minus that soluble sodium and soluble divalent cations that delivered in the irrigation water. can be used to predict the exchangeable sodium per- S. (1) A soil particle between 0.05 and 2.0 milli- centage of soil equilibrated with a given solution. De- meters in diameter. (2) A soil textural class. fined as: S . One of the components of organic soils with highly decomposed plant remains. Material is not recognizable and bulk density is low. Saprolite. Weathered rock materials that maybe soil where concentrations, denoted by parentheses, are ex- parent material. pressed in moles per liter. S . The solution extracted from a Soil. (1) The unconsolidated mineral or material on soil at its saturation water content. the immediate surface of the earth that serves as a S . A mineral resulting from the de- natural medium for the growth of land plants. (2) The composition of a primary mineral or from the reprecipi- unconsolidated mineral or organic matter on the sur- tation of the products of decomposition of a primary face of the earth, which has been subjected to and mineral. influenced by genetic and environmental factors of par- GLOSSARY 351 ent material, climate, macro- and microorganisms, and ity properties of soils per se: and those properties in topography, all acting over a period of time and pro- relation to their use and management. ducing a product-soil-that differs from the material S . Mineral particles, less than 2.0 milli- from which it is derived in many physical, chemical, meters in equivalent diameter, ranging between speci- biological, and morphological properties and charac- fied size limits. teristics. S . The lowest category in the U.S. system of S . A kind of map unit used in soil soil taxonomy: a conceptualized class of soil bodies surveys comprised of delineations, each of which (polypedons) that have limits and ranges more re- shows the size, shape, and location of a landscape unit strictive than all higher taxa. The soil series serves as a composed of two or more kinds of component soils, or major vehicle to transfer soil information and research component soils and miscellaneous areas, plus allow- knowledge from one soil area to another. able inclusions in either case. S . The aqueous liquid phase of the soil S . A combination of all manage- and its solutes. ment and land-use methods that safeguard the soil S . The combination or arrangement of against depletion or deterioration by natural or human- primary soil particles into secondary particles, units, or induced factors. peds. S . The mode of origin of the soil with S . The systematic examination, descrip- special reference to the processes or soil-forming fac- tion, classification, and mapping of soils in an area. tors responsible for development of the solum, or true S . The relative proportions of the various soil, from unconsolidated parent material. soil separates in a soil. S . A layer of soil or soil material approxi- S (). The amount of work mately parallel to the land surface and differing from that must be done per unit quantity of pure water in adjacent genetically related layers in physical, chemi- order to transport reversibly and isothermally an infini- cal, and biological properties or characteristics such as tesimal quantity of water from a pool of pure water, at a color, structure, texture, consistency, kinds and num- specified elevation and at atmospheric pressure, to the ber of organisms present, degree of acidity or alkalinity, soil water (at the point under consideration). and so on. Solum. The upper and most weathered part of the S . (1) The maximum average an- soil profile; the A, E, and B horizons. nual soil loss that will allow continuous cropping and maintain soil productivity without requiring additional S . A mineral soil horizon that is char- management imputs. (2) The maximum soil erosion acterized by the illuvial accumulation of amorphous loss that is offset by the theoretical maximum rate of materials composed of aluminum and organic carbon soil development, which will maintain an equilibrium with or without iron. between soil losses and gains. Spodosols. Mineral soils that have a spodic or a S . Groups of taxonomic placic horizon that overlies a fragipan. A soil order. soil units with similar adaptations or management re- S . The practice of growing crops that quirements for one or more specific purposes, such as require different types of tillage, such as row and sod, in adapted crops or crop rotations, drainage practices, alternate strips along contours or across the prevailing fertilization, forestry, and highway engineering. direction of the wind. S . A vertical section of a soil profile re- S . The charge (usually negative) on moved from the soil and mounted for display or study. a mineral caused by isomorphous substitution within S . The capacity of a soil to produce a the mineral layer. certain yield of crops, or other plants, with optimum Subsoiling. Any treatment to loosen soil below the management. tillage zone without inversion and with a minimum of S . That science dealing with soils as a mixing with the tilled zone. natural resource on the surface of the earth, including S . The excess of negative or soil formation, classification and mapping, geography positive charge per unit area of surface area of soil or and use, and physical, chemical, biological, and fertil- soil mineral. 352 GLOSSARY

Thermic. A soil temperature regime that has mean temperate subhumid or semiarid regions, or in tropical annual soil temperatures of 15° C or more, but less than and subtropical regions with a monsoon climate. A 22° C and more than 5° C difference between mean limited amount of water is available for plants but oc- summer and mean winter soil temperatures at a 50- curs at times when the soil temperature is optimum for centimeter depth below the surface. plant growth. Thermophile. An organism that grows readily at temperatures above 45° C. V, . The relative lightness or intensity of color and approximately a function of the square root of Till. (1) Unstratified glacial drift deposited by ice and the total amount of light. One of the three variables of consisting of clay, silt, sand, gravel, and boulders, inter- color. mingled in any proportion. (2) To prepare the soil for seeding; to seed or cultivate the soil. Vermiculite. A highly charged layer-structured silicate of the 2:1 type that is formed from mica. Tilth. The physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its impedance Vertisols. Mineral soils that have 30 percent or more to seedling emergence and root penetration. clay, deep wide cracks when dry, and either gilgai microrelief, intersecting slickensides, or wedge-shaped T . An application of fertilizer to a soil structural aggregates tilted at an angle from the horizon. surface, without incorporation, after the crop stand has A soil order. been established. V . A common endomycorrhizal Toposequence. A sequence of related soils. The association produced by phycomycetous fungi of the soils differ, one from the other, primarily because of family Endogonaceae. Host range includes most agri- topography as a soil-formation factor. cultural and horticultural crops. Torric. A soil-moisture regime defined like aridic moisture regime but used in a different category of the Waterlogged. Saturated or nearly saturated with U.S. soil taxonomy. water. Truncated. Having lost all or part of the upper soil W . See Soil water potential. horizon or horizons. W . The upper surface of groundwater or Tuff. Volcanic ash usually more or less stratified and that level in the ground where the water is at atmo- in various states of consolidation. spheric pressure. W , . The water table of a saturated Udic. A soil moisture regime that is neither dry for as layer of soil that is separated from an underlying satu- long as 90 cumulative days nor for as long as 60 consec- rated layer by an unsaturated layer (vadose water). utive days in the 90 days following the summer solstice W . See Permanent wilting point. at periods when soil temperature at 50 centimeters below the surface is above 5° C. Xeric. A soil moisture regime common to Mediterra- U . Individual soil particles after a nean climates having moist, cool winters and warm, dry standard dispersing treatment. summers. A limited amount of water is present but does Ultisols. Mineral soils that have an argillic or kan- not occur at optimum periods for plant growth. Irri- dic horizon with a basic cation saturation of less than gation or summer fallow is commonly necessary for 35 percent when measured at pH 8.2. Ultisols have a crop production. mean annual soil temperature of 8° C or higher. A soil Xerophytes. Plants that grow in or on extremely dry order. soils. U . A surface layer of mineral soil that has the same requirements as the mollic epipedon Yield. The amount of a specified substance produced with respect to color, thickness, organic carbon con- (e.g., grain, straw, total dry matter) per unit area. tent, consistence, structure, and phosphorus content, but that has a basic cation saturation less than 50 per- Z . The pH value of a solution in cent when measured at pH 7. equilibrium with a particle whose net charge from all Ustic. A soil moisture regime that is intermediate be- sources is zero. tween the aridic and udic regimes and common in Z . See No-tillage system. I NDEX

353 354 INDEX I NDEX 355 356 I NDEX I NDEX 357 358 I NDEX I NDEX 359 360 I NDEX

COLOR PLAT[;

Left is nitrogen-deficient section of corn leaf and cut open section of stem that remains uncolored when treated with diaphenylamine, indicating low nitrate level in plant sap . On right the leaf is dark green and diaphenylamine produces a dark blue color indicative of high nitrate level in plant sap .

Nitrogen deficiency on corn . Lower leaves turn yellow along midrib, starting at the leaf tip .

Potassium deficiency on corn . Lower leaves have yellow margins .

Potassium deficiency on alfalfa (and clovers) shows as a series of white dots near leaf margins ; in advanced stages entire leaf margin turns white . (Courtesy American Potash Institute .)

COLOR PLATE 2

Phosphorus deficiency on corn is indicated by purplish discoloration .

Iron deficiency on pin oak . The leaf veins remain green as the intervein areas lose their green color .

Iron deficiency on roses . Green veins with yellow intervein areas, showing most on newest leaves . --C^ COLOR PLATE 3

Manganese deficiency on kidney beans . Leaf veins remain green as the intervein areas lose their green color and turn yellow .

Zinc deficiency of navy beans grown on calcareous soil . The small unfertilized zinc-deficient plants stand in marked contrast to the taller plants that were fertilized with zinc . A case where zinc fertilization is necessary to produce a crop .

Boron deficiency on sugar beets causes heart rot . Most advanced symptom is on left . COLOR PLATE 4

Magnesium deficiency on coffee . Veins remain green and intervein areas turn yellow .

Manganese-treated plants in rear showing no deficiency . Plants in foreground are unfertilized and difference in degree of manganese deficiency symptoms is indicative of varietal response to limited soil manganese .

Excess soluble salt symptoms on geranium . The leaf margins turn yellow and become necrotic . to w f- J CL I 0 J 0